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/main.py

@@ -0,0 +1,37 @@
+"""FastAPI app for the yunke data-mirror viewing API.
+
+Run locally with::
+
+    uvicorn app.api.main:app --reload
+
+Includes:
+* ``GET /api/health``  -> ``{"status": "ok"}`` liveness probe.
+* ``GET /api/customers`` (see :mod:`app.api.routers.customer`).
+
+CORS is configured permissively (wildcard origin, no credentials) so the local
+frontend can call the API during development. Auth / finer CORS are LATER tasks.
+"""
+
+from __future__ import annotations
+
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+from app.api.routers import customer
+
+app = FastAPI(title="yunke data mirror")
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["*"],
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+app.include_router(customer.router)
+
+
+@app.get("/api/health")
+async def health() -> dict[str, str]:
+    """Trivial liveness probe."""
+    return {"status": "ok"}