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.

backend/src/__tests__/image-update-service.test.ts

@@ -385,8 +385,9 @@ services:
     expect(mockDispatchAlert).toHaveBeenCalledTimes(1);
     expect(mockDispatchAlert).toHaveBeenCalledWith(
       'info',
+      'image_update_available',
       expect.stringContaining('stackA'),
-      'stackA',
+      { stackName: 'stackA' },
     );
     expect(mockUpsertStackUpdateStatus).toHaveBeenCalledWith(1, 'stackA', true, expect.any(Number));
   });
@@ -413,7 +414,7 @@ services:
     await (service as any).checkNode(1, 'local', fakeDb());
 
     expect(mockDispatchAlert).toHaveBeenCalledTimes(2);
-    const dispatched = mockDispatchAlert.mock.calls.map(call => call[2]);
+    const dispatched = mockDispatchAlert.mock.calls.map(call => (call[3] as any)?.stackName);
     expect(dispatched).toEqual(expect.arrayContaining(['stackA', 'stackB']));
     expect(mockSetSystemState).toHaveBeenCalledWith('image_update_notifications_backfilled', '1');