merge(deferred-10): aggregated live alarm stream for Alarm Summary

Merges plan #10 (feat/live-alarm-stream) into main alongside plan #22.

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

docs/requirements/Component-CentralUI.md

@@ -187,10 +187,11 @@ Per-leaf alarm rendering (leaf nodes are individual conditions for native alarms
 
 ### Alarm Summary (Deployment Role)
 - A dedicated operator **Alarm Summary** page (`/monitoring/alarms`, `RequireDeployment`) gives a **cross-instance, read-only** roll-up of live alarm state at a site — the operator-facing complement to the per-instance Debug View.
-- **Data path** — no new site-side code and no central alarm store. The page selects a site, queries its deployed instances, then fans out the existing per-instance `DebugViewSnapshot` Ask **concurrently** (capped with a `SemaphoreSlim`) and aggregates the returned `AlarmStates` client-side. The fan-out is **partial-results tolerant**: instances that time out are listed as "not reporting" while the rest still render.
+- **Data path** — no new site-side code and no central alarm store. The page selects a site, queries its deployed instances, then fans out the existing per-instance `DebugViewSnapshot` Ask **concurrently** (capped with a `SemaphoreSlim`) and aggregates the returned `AlarmStates` client-side. The fan-out is **partial-results tolerant**: instances that time out are listed as "not reporting" while the rest still render. This snapshot fan-out now doubles as the **seed** for a near-real-time live feed (see **Live updates** below) rather than the sole refresh mechanism.
+- **Live updates** — the page is driven by a **transient, per-site central live alarm cache** (`ISiteAlarmLiveCache`, owned by the Communication component; see [Component-Communication](Component-Communication.md)). On site select the page subscribes to the cache; the cache runs one shared, reference-counted per-site aggregator that **seeds** from the snapshot fan-out and then stays warm on a single **site-wide, alarm-only** `SubscribeSite` gRPC stream (seed-then-stream, dedup by `(InstanceUniqueName, AlarmName, SourceReference)`). Applied deltas raise an in-process change event (mirroring `IDeploymentStatusNotifier`) that the Blazor circuit pushes to the browser via `StateHasChanged()` — no new SignalR hub. `AlarmSummaryService.BuildFromLiveAlarms` rebuilds the roll-up + rows from the cache's current alarm set. The cache is **purely in-memory on the active central node** — there is still **no persisted central alarm store**; on a NodeA↔NodeB failover the new active node re-seeds from scratch.
 - **View** — roll-up tiles (total active, worst severity, unacked count, per-`AlarmKind` counts) plus a flat, sortable, filterable table. Filters cover instance, `AlarmKind` (Computed / NativeOpcUa / NativeMxAccess), state, acked/unacked, severity threshold, and name search.
 - **Read-only** — there are no ack / shelve / suppress controls (native alarms remain read-only by design).
-- **Refresh** — manual refresh button plus an optional poll timer (mirroring the Health dashboard); there is no aggregated live alarm stream in this release (snapshot + poll only — logged as a follow-up).
+- **Refresh** — manual refresh button plus the 15s poll timer (mirroring the Health dashboard), now retained as a **fallback + `NotReporting` authority** behind the live cache: when the cache reports `IsLive`, the page renders live-cache state; when a stream is unhealthy or a site has not yet seeded, the poll keeps the page fresh so a stream failure never blanks it. (Aggregated live stream **delivered 2026-07-10** — see `docs/plans/2026-07-10-aggregated-live-alarm-stream-plan.md`.)
 - **Reuse** — the alarm badge/formatter markup is factored out of Debug View into a shared `AlarmStateBadges` component consumed by both Debug View and this page.
 
 ### Parked Message Management (Deployment Role)

docs/requirements/Component-Communication.md

@@ -67,6 +67,17 @@ Both central and site clusters. Each side has communication actors that handle m
 - **Accepted limitation (central-side session locality):** debug sessions are process-local state on the central node hosting the `DebugStreamBridgeActor`. A central **restart or failover drops all active sessions** with no `OnStreamTerminated` signal to the operator — the engineer simply re-establishes the debug session from the UI. Debug streaming is an interactive, transient diagnostic aid, so this is an accepted trade-off rather than a durability gap (arch review 02, U7).
 - **Backpressure is lossy-by-design and now observable:** the per-session `Channel<SiteStreamEvent>` (bounded 1000, `DropOldest`) silently evicts the oldest event when a slow consumer falls behind. The eviction is counted via the channel's `itemDropped` callback and logged at Warning (first eviction, then every 500th) so real event loss on a debug stream is visible instead of silent.
 
+### 6.1 Aggregated Live Alarm Stream (Site → Central)
+
+Delivered 2026-07-10 (`docs/plans/2026-07-10-aggregated-live-alarm-stream-plan.md`) for the operator Alarm Summary page (Central UI #9). Where §6 streams **one instance** for the interactive Debug View, this feeds a **transient, per-site central live alarm cache** so the Alarm Summary reflects alarm transitions in near-real-time instead of re-polling the whole site every 15s.
+
+- **Site-wide, alarm-only stream**: a single `SubscribeSite` gRPC stream per site carries `AlarmStateChanged` events for **all** instances (attribute events are dropped — the summary never shows them, and attributes are far higher-volume). On the site this reuses the existing `SiteStreamManager` broadcast hub via `SiteStreamManager.SubscribeSiteAlarms(...)` (same wiring as the per-instance `Subscribe`, minus the `InstanceUniqueName` filter and filtered to alarms). `StreamRelayActor` is reused unchanged, so placeholder rows (`is_configured_placeholder`) are still dropped at the relay.
+- **Central live cache** (`ISiteAlarmLiveCache`, singleton `SiteAlarmLiveCacheService`): a DI singleton on the active central node. For each site with ≥1 active viewer it runs ONE shared, **reference-counted** per-site aggregator (`SiteAlarmAggregatorActor`); the first `Subscribe(siteId, onChanged)` starts it, the last subscriber leaving stops it after a short **linger** to avoid re-seed thrash. `GetCurrentAlarms(siteId)` returns the current immutable snapshot; `IsLive(siteId)` reports whether the aggregator has seeded and published at least once.
+- **Seed-then-stream** (copied from `DebugStreamBridgeActor` ordering): open the `SubscribeSite` stream first (buffer live deltas), run the snapshot fan-out once via the existing `DebugViewSnapshot` path (bounded by `LiveAlarmCacheSeedConcurrency`), flush the buffer with **dedup by `(InstanceUniqueName, AlarmName, SourceReference)`**, then live pass-through. Placeholders are seeded from the snapshot and never expected on the live stream.
+- **Failover & drift**: NodeA↔NodeB reconnect (client via `SiteStreamGrpcClient.SubscribeSiteAsync`, factory endpoint failover) re-seeds rather than serving stale; a periodic **reconcile snapshot** (default 60s) corrects any missed enable/disable so the cache can't drift indefinitely. `[PERM]` (`docs/plans/2026-05-29-native-alarms-design.md`): the cache is **purely in-memory** — no EF entity/table/migration, no persisted central alarm store — so a new active node simply re-seeds from scratch.
+- **Options** (`Communication` section, `CommunicationOptions`; eagerly validated by `CommunicationOptionsValidator` / `ValidateOnStart`): `LiveAlarmCacheLinger` (default 30s), `LiveAlarmCacheReconcileInterval` (default 60s), `LiveAlarmCacheSeedConcurrency` (default 8), `LiveAlarmCacheMaxSubscribersPerSite` (default 200).
+- **Telemetry** (`ScadaBridgeTelemetry` meter): observable gauge `scadabridge.site.alarm_cache.aggregators.active` (running per-site aggregators) and counter `scadabridge.site.alarm_cache.reconnects` (site-wide stream reconnects — a NodeA↔NodeB flip or reconcile-driven reopen; a sustained climb signals a flapping site link).
+
 #### Site-Side gRPC Streaming Components
 
 - **SiteStreamGrpcServer**: gRPC service (`SiteStreamService.SiteStreamServiceBase`) hosted on each site node via Kestrel HTTP/2 on a dedicated port (default 8083). Implements the `SubscribeInstance` RPC. For each subscription, creates a `StreamRelayActor` that subscribes to `SiteStreamManager`, bridges events through a `Channel<SiteStreamEvent>` to the gRPC response stream. Tracks active subscriptions by `correlation_id` — duplicate IDs cancel the old stream. Enforces a max concurrent stream limit (default 100). Rejects streams with `StatusCode.Unavailable` before the actor system is ready.
@@ -84,13 +95,14 @@ Both central and site clusters. Each side has communication actors that handle m
 
 The streaming protocol is defined in `sitestream.proto` (`src/ZB.MOM.WW.ScadaBridge.Communication/Protos/sitestream.proto`):
 
-- **Service**: `SiteStreamService` — hosted on each site node by `SiteStreamGrpcServer` — exposes five RPCs. One is the original real-time **server-streaming** subscription; the other four are **unary request/response** calls added by the Audit Log (#23) and Site Call Audit (#22) components. A unary call is request/response and is distinct from the command/control ClusterClient channel — gRPC on this service is no longer real-time-stream-only:
-  - `SubscribeInstance(InstanceStreamRequest) returns (stream SiteStreamEvent)` — the real-time debug stream (§6); the only server-streaming RPC.
+- **Service**: `SiteStreamService` — hosted on each site node by `SiteStreamGrpcServer` — exposes six RPCs. Two are real-time **server-streaming** subscriptions (`SubscribeInstance`, `SubscribeSite`); the other four are **unary request/response** calls added by the Audit Log (#23) and Site Call Audit (#22) components. A unary call is request/response and is distinct from the command/control ClusterClient channel — gRPC on this service is no longer real-time-stream-only:
+  - `SubscribeInstance(InstanceStreamRequest) returns (stream SiteStreamEvent)` — the per-instance real-time debug stream (§6).
+  - `SubscribeSite(SiteStreamRequest) returns (stream SiteStreamEvent)` — the **site-wide, alarm-only** aggregated stream (§6.1) added for the operator Alarm Summary live cache. Additive (new RPC + new `SiteStreamRequest { correlation_id }` message; no field renumbering). The server handler mirrors `SubscribeInstance` — same bounded `Channel(1000, DropOldest)`, `GrpcMaxConcurrentStreams`/`GrpcMaxStreamLifetime` limits, `SiteConnectionOpened/Closed` telemetry, and `StreamRelayActor` mapping — but subscribes via `SiteStreamManager.SubscribeSiteAlarms` (all instances, `AlarmStateChanged` only; attribute events dropped, no per-instance filter).
   - `IngestAuditEvents(AuditEventBatch) returns (IngestAck)` — central-side **ingest** receiving surface for Audit Log (#23) telemetry; routes the batch to the central `AuditLogIngestActor` proxy and returns the accepted `EventId`s. (The production *push* path is still ClusterClient via `ClusterClientSiteAuditClient`; this RPC is the gRPC-receiving counterpart.)
   - `IngestCachedTelemetry(CachedTelemetryBatch) returns (IngestAck)` — ingest receiving surface for the combined cached-call telemetry packet (audit row + `SiteCalls` operational upsert written in one transaction).
   - `PullAuditEvents(PullAuditEventsRequest) returns (PullAuditEventsResponse)` — central→site **reconciliation pull** for the Audit Log self-heal feed; the site serves `Pending`/`Forwarded` rows from its `ISiteAuditQueue`.
   - `PullSiteCalls(PullSiteCallsRequest) returns (PullSiteCallsResponse)` — central→site reconciliation pull for the Site Call Audit (#22) self-heal feed; the site serves operation-tracking rows changed since a cursor from its `IOperationTrackingStore`. A separate RPC from `PullAuditEvents` because the tracking store is the operational source of truth, distinct from the site audit queue.
-- **Messages**: `InstanceStreamRequest` (correlation_id, instance_unique_name), `SiteStreamEvent` (correlation_id, oneof event: `AttributeValueUpdate`, `AlarmStateUpdate`); `AuditEventDto`/`AuditEventBatch`/`IngestAck` for ingest; `CachedTelemetryPacket`/`CachedTelemetryBatch` (each packet pairing an `AuditEventDto` with a `SiteCallOperationalDto`); `PullAuditEventsRequest`/`PullAuditEventsResponse` and `PullSiteCallsRequest`/`PullSiteCallsResponse` (each request carries `since_utc` + `batch_size`; each response carries `more_available` to signal a saturated batch).
+- **Messages**: `InstanceStreamRequest` (correlation_id, instance_unique_name), `SiteStreamRequest` (correlation_id only — no instance name; drives the site-wide `SubscribeSite` stream), `SiteStreamEvent` (correlation_id, oneof event: `AttributeValueUpdate`, `AlarmStateUpdate`); `AuditEventDto`/`AuditEventBatch`/`IngestAck` for ingest; `CachedTelemetryPacket`/`CachedTelemetryBatch` (each packet pairing an `AuditEventDto` with a `SiteCallOperationalDto`); `PullAuditEventsRequest`/`PullAuditEventsResponse` and `PullSiteCallsRequest`/`PullSiteCallsResponse` (each request carries `since_utc` + `batch_size`; each response carries `more_available` to signal a saturated batch).
 - The `oneof event` pattern is extensible — future event types (health metrics, connection state changes) are added as new fields without breaking existing consumers.
 - Proto field numbers are never reused; new RPCs and message fields are appended additively. Old clients ignore unknown `oneof` variants.