feat: ship the AI assistant as a first-party in-process plugin

Moves src/backend/ai and its routes into plugins/ai, mounted through a
small /ai dispatcher instead of its own port. Moves the ai-enabled and
ai-private-endpoints settings routes out of user-settings-routes.ts
into the plugin too, so it has no leftover core dependents.

tsconfig.plugins.json

@@ -28,7 +28,8 @@
   },
   "include": [
     "plugins/docker/backend/**/*.ts",
-    "plugins/host-metrics/backend/**/*.ts"
+    "plugins/host-metrics/backend/**/*.ts",
+    "plugins/ai/backend/**/*.ts"
   ],
   "exclude": ["src/**/*.test.ts"]
 }