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>

infra/prod/env.example

@@ -16,6 +16,8 @@ LANDING_URL=https://barevclub.com
 CLIENT_MINIAPP_URL=https://app.barevclub.com
 PRO_MINIAPP_URL=https://pro.barevclub.com
 ADMIN_URL=https://admin.barevclub.com
+# новая админка — когда выложим: тогда «Открыть» в уведомлениях о сообщениях ведёт в её чаты
+# BACKOFFICE_URL=https://team.barevclub.com
 # серверный адрес API для лендинга (внутри docker-сети)
 API_URL=http://barev-app:4000
 API_PORT=4000