feat(notifications): add shared notification routing rules (Admiral tier) (#347)

Route stack alerts to specific Discord, Slack, or webhook channels instead
of the single global endpoint. Includes per-rule enable/disable, priority
ordering, and automatic fallback to global agents when no rule matches.

- Add notification_routes table, interface, and CRUD in DatabaseService
- Add routing logic in NotificationService.dispatchAlert with optional stackName
- Pass stack context from MonitorService (crash/health) and SchedulerService
- Add 5 API endpoints gated with requireAdmin + requireAdmiral
- Add NotificationRoutingSection UI with Combobox stack picker, channel tabs
- Parallel webhook dispatch via Promise.allSettled
- 10 unit tests covering routing, fallback, and edge cases
- Documentation with screenshots at docs/features/notification-routing.mdx

docs/docs.json

@@ -105,6 +105,7 @@
               "features/fleet-view",
               "features/stack-labels",
               "features/alerts-notifications",
+              "features/notification-routing",
               "features/webhooks",
               "features/rbac",
               "features/atomic-deployments",