Run AI classification in background with SignalR notifications

Move the classify-uncategorized endpoint from synchronous to async.
The controller now queues a ClassificationJob to an unbounded channel
and returns 202 Accepted. A new BackgroundService processes jobs and
pushes results (or errors) to the user via SignalR. The frontend
listens for the AiClassificationComplete event and surfaces it in the
notification system.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/composables/useNotifications.ts

@@ -1,7 +1,7 @@
 import { ref, computed } from 'vue'
 import { HubConnectionBuilder, HubConnection, LogLevel } from '@microsoft/signalr'
 import { useAuthStore } from '@/stores/auth'
-import type { AppNotification, FileImportNotification, BankSyncNotification } from '@/types'
+import type { AppNotification, FileImportNotification, BankSyncNotification, AiClassificationNotification } from '@/types'
 
 const notifications = ref<AppNotification[]>([])
 const isConnected = ref(false)
@@ -42,6 +42,16 @@ function connect() {
     })
   })
 
+  connection.on('AiClassificationComplete', (data: AiClassificationNotification) => {
+    notifications.value.unshift({
+      id: crypto.randomUUID(),
+      type: 'AiClassificationComplete',
+      data,
+      timestamp: new Date().toISOString(),
+      read: false
+    })
+  })
+
   connection.onclose(() => {
     isConnected.value = false
   })