Backoffice stage 2: chats (support + concierge, files)

- ChatThread per client/provider and channel; messages get files, internal notes,
  source (bot/mini app) and delivery errors; reply templates. Migration moves old
  concierge messages into threads.
- Routing: subscribers write to the concierge, others to support; /support and
  /concierge in the client bot; staff can take, transfer, close and move threads.
- Telegram: photos/documents from clients and providers, replies with the channel
  header, files sent from storage, notifications only to the assignee or the
  channel's staff, "Reply" into a specific thread.
- New admin: /chats with list, conversation, notes, files, templates, client panel;
  "Message" button on the client card; dashboard counts threads awaiting reply.
- Client mini app: in-app chat screen sharing the same conversation.
- Old admin keeps working on the same data (notes hidden there).
- Tests for routing, notes, assign/close/move, lists, recipients; separate test DB
  via TEST_DB_NAME; dev:chats-demo script.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/api/package.json

@@ -14,7 +14,8 @@
     "typecheck": "tsc --noEmit",
     "dev:initdata": "tsx scripts/dev-initdata.ts",
     "dev:demo": "tsx scripts/dev-demo.ts",
-    "dev:bo-demo": "tsx scripts/dev-backoffice-demo.ts"
+    "dev:bo-demo": "tsx scripts/dev-backoffice-demo.ts",
+    "dev:chats-demo": "tsx scripts/dev-chats-demo.ts"
   },
   "dependencies": {
     "@aws-sdk/client-s3": "^3.1152.0",