feat(admin): monitoring center

Sprint 26.

New features/admin/monitoring/ module + net-new /:lang/backoffice/monitoring
route + Dashboard Quick Action.

- Health section reuses AdminDashboardFacade.healthChecks directly (real
  data, unchanged since Sprint 19) instead of duplicating the logic
- unified AdminMonitoringEvent feed covering audit/security/login/
  failed-login/api/error/warning, category filter + search, 40 seeded
  synthetic entries (no logging backend exists anywhere in this system)
- mock queue depth/status cards, mock webhook delivery log
- intentionally kept separate from Sprint 24's per-transaction audit and
  Sprint 25's per-user audit - different scopes, no consolidation attempted

Also fixed a real type error: AdminDashboardQuickActionId's union was
missing 'users' and 'monitoring' (added when wiring those Quick Actions),
caught by ng build's template type-checking even though plain tsc --noEmit
passed - a reminder that ng build is the authoritative check here.

docs/ADMIN.md + docs/BACKEND.md (new item 15) updated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/ADMIN.md

@@ -369,6 +369,28 @@ routes at all, this is a net-new admin section.
 - Wired into `AdminDashboardFacade`'s Quick Actions list (`dashboard.actionUsers`
   -> `/​:lang/backoffice/users`).
 
+## Sprint 26 - Monitoring (mock/local, health reuses real data)
+
+`features/admin/monitoring/`, single page at `/​:lang/backoffice/monitoring`
+(new Dashboard Quick Action).
+
+- **Health**: reuses `AdminDashboardFacade.healthChecks` directly (the same
+  real, non-mocked bootstrap-validation checks from Sprint 19's dashboard)
+  instead of duplicating the logic - this is the one section on this page
+  backed by real data.
+- **Audit / security / login / failed-login / API / error / warning
+  events**: one unified `AdminMonitoringEvent` feed (`category` + `level`
+  discriminators) with category filter + search, seeded with 40
+  deterministic synthetic entries by `AdminMonitoringLocalGateway` - no
+  logging backend exists anywhere in this system, so there is nothing real
+  to read from.
+- **Queue monitoring**: 3 mock named queues with depth + status.
+- **Webhook monitoring**: mock delivery log (endpoint/event/status/time).
+- This is deliberately a separate, system-wide log from the two
+  narrower-scoped audit trails added earlier: Sprint 24's per-transaction
+  audit and Sprint 25's per-user audit. No consolidation attempted - they
+  track different things.
+
 ## Known gaps / backend needs
 
 - **Dashboard metrics endpoint.** Categories/Products counts are computed