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/services/ImageUpdateService.ts

@@ -275,8 +275,9 @@ export class ImageUpdateService {
                 try {
                     await notifier.dispatchAlert(
                         'info',
+                        'image_update_available',
                         `[Node: ${nodeName}] Stack "${stackName}" has image updates available.`,
-                        stackName,
+                        { stackName },
                     );
                 } catch (e) {
                     console.error(`[ImageUpdateService] Failed to dispatch update notification for "${stackName}":`, e);
@@ -286,6 +287,7 @@ export class ImageUpdateService {
                     try {
                         db.addNotificationHistory(NodeRegistry.getInstance().getDefaultNodeId(), {
                             level: 'error',
+                            category: 'system',
                             message: `[Node: ${nodeName}] Failed to notify about image updates for stack "${stackName}": ${getErrorMessage(e, String(e))}`,
                             timestamp: Date.now(),
                         });