feat: customer list API

Minimal read-only FastAPI surface over the mirrored DB:
- GET /api/customers with pagination (page/page_size) and optional
  channel / owner_account_id / department_id filters, ordered by
  last_chat_time desc (nulls last) then friend_id.
- GET /api/health liveness probe.
- Pydantic v2 CustomerOut + generic Page[T] envelope.
- CORS configured permissively for the local frontend.
- Tests override get_session to an in-memory async SQLite session
  (StaticPool) and drive the app via httpx ASGITransport.

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

yunke-server/app/api/__init__.py

@@ -0,0 +1,6 @@
+"""FastAPI layer for the yunke data-mirror viewing API.
+
+Read-only HTTP surface over the mirrored DB. Public entrypoint is
+:mod:`app.api.main` (``app = FastAPI(...)``). Routers live under
+:mod:`app.api.routers` and share dependencies from :mod:`app.api.deps`.
+"""