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/BACKEND.md

@@ -160,6 +160,14 @@ Plus, if authenticated history/wishlist/compare/saved-searches sync is wanted: `
 
 **Frontend files:** implement `AdminUsersApiGateway` against `AdminUsersGateway` (`services/admin-users-gateway.interface.ts`) and rebind via an injection token.
 
+## 15. Monitoring (Sprint 26, mock/local except Health)
+
+**Current behavior:** `features/admin/monitoring/` — Health section reads real data (`AdminDashboardFacade.healthChecks`, unchanged from Sprint 19). Everything else (audit/security/login/failed-login/API/error/warning event feed, queue depths, webhook deliveries) is synthetic, seeded once in `AdminMonitoringLocalGateway` — no logging, queue, or webhook infrastructure exists anywhere in this system.
+
+**Needed:** real structured logging with a query API (by category/level/actor/time range), real queue introspection (whatever job runner ships), and real webhook delivery tracking once webhooks exist as a feature at all.
+
+**Frontend files:** implement `AdminMonitoringApiGateway`-equivalent methods against a to-be-defined `AdminMonitoringGateway` interface (`AdminMonitoringLocalGateway` currently has no interface extracted — add one when a real implementation is built, mirroring the pattern used everywhere else in `admin/*`).
+
 ## Known reliability issues
 
 ### Production 502/504 Bad Gateway on refresh / back-navigation