feat(notifications): add structured category enum to dispatcher and history (#774)

Introduce a NotificationCategory string-literal union (11 values) and
thread it through dispatchAlert as a required second argument. All
callers (DockerEventService, AutoHealService, ImageUpdateService,
MonitorService, PolicyEnforcement, policyGate, SchedulerService,
imageUpdates route) pass an explicit category at every call site,
giving TypeScript compile-time enforcement that no new emit site can
be added without choosing a category.

DatabaseService gains an idempotent migration that adds a nullable
category TEXT column to notification_history; existing rows keep
category=NULL (displayed as Uncategorized in the UI). The
getNotificationHistory method accepts an optional category filter
that is forwarded from the GET /api/notifications/history route via
a ?category= query param.

NotificationPanel gains a category Select dropdown so users can
filter history by category. The frontend types mirror the backend
union so API responses are type-safe end-to-end.

All 75 test files (1410 tests) updated to the new 4-arg dispatchAlert
signature and passing.

frontend/src/components/dashboard/types.ts

@@ -43,9 +43,23 @@ export interface MetricPoint {
   net_tx_mb: number;
 }
 
+export type NotificationCategory =
+    | 'deploy_success'
+    | 'deploy_failure'
+    | 'stack_started'
+    | 'stack_stopped'
+    | 'stack_restarted'
+    | 'image_update_available'
+    | 'image_update_applied'
+    | 'autoheal_triggered'
+    | 'monitor_alert'
+    | 'scan_finding'
+    | 'system';
+
 export interface NotificationItem {
   id: number;
   level: 'info' | 'warning' | 'error';
+  category?: NotificationCategory | string;
   message: string;
   timestamp: number;
   is_read: number;