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/deps.py

@@ -0,0 +1,10 @@
+"""FastAPI dependencies for the yunke data-mirror API.
+
+Re-exports :func:`app.db.base.get_session` so route code (and tests) depend on
+a single API-layer seam. Tests override ``app.api.deps.get_session`` to swap in
+an isolated in-memory SQLite session.
+"""
+
+from app.db.base import get_session
+
+__all__ = ["get_session"]