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/routes/notifications.ts

@@ -25,7 +25,8 @@ export const notificationsRouter = Router();
 notificationsRouter.get('/', authMiddleware, async (req: Request, res: Response): Promise<void> => {
   try {
     const nodeId = req.nodeId ?? 0;
-    const history = DatabaseService.getInstance().getNotificationHistory(nodeId);
+    const category = typeof req.query.category === 'string' ? req.query.category : undefined;
+    const history = DatabaseService.getInstance().getNotificationHistory(nodeId, 50, category);
     res.json(history);
   } catch {
     res.status(500).json({ error: 'Failed to fetch notifications' });