feat(notifications): add notification center with real-time updates

- add notifications DB schema, repository, service, gateway, and controller
- add shared notification types, categories, and preference models
- add NotificationAccess permission with authorization matrix probe
- add frontend composable with Socket.IO subscription and REST API
- add NotificationPopover, NotificationItem, and NotificationPreferences components
- integrate notification producers in scanner, metadata fetch, book bucket, author enrichment, email, migration, and file write-back services
- add 30-day auto-cleanup via existing CleanupService cron
- add 29 backend unit tests for notification service and controller

server/test/authorization-matrix.e2e-spec.ts

@@ -411,6 +411,11 @@ describe('Authorization matrix (e2e)', () => {
           path: '/audit-log',
           token: 'allPerms',
         },
+        [Permission.NotificationAccess]: {
+          method: 'GET',
+          path: '/notifications',
+          token: 'allPerms',
+        },
       };
 
       const failures: MatrixFailure[] = [];