feat(health): surface pending-updates counts on the Health dashboard and CLI (ScadaBridge#63)

Health page gains PendingUpdatesHealthTiles (stale instances / sites with stale
artifacts, Deployer+Administrator only, site-scope narrowed, shared cache). New
GetPendingUpdates management command (Deployer) and 'health pending-updates' CLI
leaf. Flatten/cache computation untouched; the command reads
IPendingUpdatesEvaluator directly because the UI cache is not reachable from
ManagementService.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

docs/requirements/Component-HealthMonitoring.md

@@ -42,6 +42,8 @@ Site clusters (metric collection and reporting). Central cluster (aggregation an
 | `SecretMirrorStatuses` | Site Runtime (`DeployedSecretsMirror` via the replicated `secret_mirror_status` table) | Every site node's deployed-secrets mirror status — node name, when the node last wrote the row (on a failure-set change, at start-up, or on the 10-min refresh), and its current failures (`{SecretName, Message}`; names and messages only, never values). Review finding M1: the **standby's** failures ride the **active** node's report, because each node writes its own row to a LocalDb-replicated table and the active node's `SecretMirrorStatusReporter` reads them all. **Nullable — null = no data** (central self-report, a pre-M1 site, reporter not yet run); an empty list = wired, no row yet; a node with an empty `Failures` list is healthy. Point-in-time (not drained on collect). Wire: `SiteHealthReportDto.secret_mirror_statuses` (field **33**, a presence wrapper so null ≠ empty). Drives the *Secret mirror failing* dashboard badge + per-node failure list. On an unreplicated pair only the active node's own row appears. A node that has not refreshed its row for `SiteRuntime:SecretMirrorStatusStaleAfter` (default 1 h — decommissioned, renamed or down) is left off and its row deleted. |
 | `LocalDbOplogBacklog` | Consolidated site LocalDb | Unacked oplog depth (Prometheus `localdb_oplog_depth`). **Nullable — null means "no reading", NOT "connected with an empty backlog"** (a failed poll rendered as 0 would report a pair that cannot read its own oplog as perfectly healthy). **Expected transient spikes:** the daily `site_events` retention purge and the storage-cap trim are CDC-captured like any other write, so the backlog jumps by the batch size at purge time and drains as the peer acks it — see `Component-SiteEventLogging.md` → Storage and `docs/deployment/topology-guide.md` → *Reading the replication backlog* for the healthy-versus-faulty signature. |
 
+**Pending-updates tiles (Gitea #63).** The Health dashboard also shows two central-computed counts — instances out of date and sites with stale artifacts — read from the same process-wide pending-updates cache as the rail indicator (no extra fleet flatten). Visible to Deployer/Administrator only and narrowed to a site-scoped operator's permitted sites. The CLI equivalent is `scadabridge health pending-updates` (`GetPendingUpdates`, Deployer role), which evaluates the fleet directly rather than through the UI cache.
+
 ## Reporting Protocol
 
 - Sites send a **health report message** to central at a configurable interval (default: **30 seconds**).