docs(notification-outbox): add central-computed outbox KPIs to Health Monitoring

docs/requirements/Component-HealthMonitoring.md

@@ -29,8 +29,11 @@ Site clusters (metric collection and reporting). Central cluster (aggregation an
 | Tag resolution counts | Data Connection Layer | Per connection: total subscribed tags vs. successfully resolved tags |
 | Script error rates | Site Runtime (Script Actors) | Frequency of script failures |
 | Alarm evaluation error rates | Site Runtime (Alarm Actors) | Frequency of alarm evaluation failures |
-| Store-and-forward buffer depth | Store-and-Forward Engine | Pending messages by category (external, notification, DB write) |
+| Store-and-forward buffer depth | Store-and-Forward Engine | Pending messages by category — external, notification (notifications awaiting forward to central), DB write |
 | Dead letter count | Akka.NET EventStream | Messages sent to actors that no longer exist — indicates stale references or timing issues |
+| Notification Outbox queue depth | Notification Outbox (central) | Count of `Pending` + `Retrying` notifications — central-computed, not site-reported |
+| Notification Outbox stuck count | Notification Outbox (central) | Count of `Pending` / `Retrying` notifications older than the configurable stuck-age threshold — central-computed, not site-reported |
+| Notification Outbox parked count | Notification Outbox (central) | Count of `Parked` notifications — central-computed, not site-reported |
 
 ## Reporting Protocol
 
@@ -50,10 +53,22 @@ Script error rates and alarm evaluation error rates are calculated as **raw coun
 - **Alarm evaluation errors** include all failures during alarm condition evaluation.
 - For detailed diagnostics (error types, stack traces, affected instances), operators use the **Site Event Log Viewer** — the health dashboard is for quick triage, not forensics.
 
+## Notification Outbox KPIs
+
+The Notification Outbox is a **central** component, so its KPIs are **central-computed** rather than collected from sites and carried in the site health report:
+
+- The dashboard surfaces three **headline** outbox KPIs: **queue depth** (`Pending` + `Retrying`), **stuck count** (`Pending` / `Retrying` rows older than the configurable stuck-age threshold), and **parked count** (`Parked`).
+- The Notification Outbox component computes these on demand from the central `Notifications` table; the health dashboard polls it for the headline tiles.
+- The fuller KPI set — which also includes **delivered (last interval)** and **oldest pending age** — lives on the Central UI **Notification Outbox** page, not the health dashboard.
+- Outbox KPIs are **point-in-time**, computed on demand from the `Notifications` table. There is no time-series store — consistent with Health Monitoring's "current status only" philosophy. The outbox's own ~1-year row retention answers historical questions directly.
+
+These are distinct from the site-reported **Store-and-forward buffer depth** notification metric, which now covers the **site→central leg** — notifications still buffered in a site's Store-and-Forward Engine awaiting forward to central — and remains part of the site health report.
+
 ## Central Storage
 
 - Health metrics are held **in memory** at the central cluster for display in the UI.
 - No historical health data is persisted — the dashboard shows current/latest status only.
+- Notification Outbox KPIs are not stored by Health Monitoring; they are computed point-in-time from the central `Notifications` table each time the dashboard refreshes — consistent with the current-status-only philosophy.
 - Site connectivity history (online/offline transitions) may optionally be logged via the Audit Log or a separate mechanism if needed in the future.
 
 ## No Alerting
@@ -66,8 +81,9 @@ Script error rates and alarm evaluation error rates are calculated as **raw coun
 - **Communication Layer**: Transports health reports from sites to central.
 - **Data Connection Layer (site)**: Provides connection health metrics.
 - **Site Runtime (site)**: Provides script error rate and alarm evaluation error rate metrics.
-- **Store-and-Forward Engine (site)**: Provides buffer depth metrics.
+- **Store-and-Forward Engine (site)**: Provides buffer depth metrics, including the notification backlog awaiting forward to central.
 - **Cluster Infrastructure (site)**: Provides node role status.
+- **Notification Outbox (central)**: Provides central-computed outbox KPIs — queue depth, stuck count, parked count — for the headline dashboard tiles.
 
 ## Interactions