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/routers/__init__.py

@@ -0,0 +1 @@
+"""HTTP routers for the yunke data-mirror API."""

yunke-server/app/api/routers/customer.py

@@ -0,0 +1,70 @@
+"""GET /api/customers — paginated, filterable customer list.
+
+Read-only query over the mirrored ``customer`` table. Supports filtering by
+``channel``, ``owner_account_id`` (direct columns) and ``department_id``
+(resolved through ``customer.user_id`` -> ``staff.dept_id``). Results are
+ordered by ``last_chat_time desc`` (nulls last) then ``friend_id`` so the
+most-recently-active contacts surface first.
+"""
+
+from __future__ import annotations
+
+from typing import Annotated
+
+import sqlalchemy as sa
+from fastapi import APIRouter, Depends, Query
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.api.deps import get_session
+from app.api.schemas import CustomerOut, Page
+from app.db.models import Customer, Staff
+
+router = APIRouter(prefix="/api", tags=["customers"])
+
+SessionDep = Annotated[AsyncSession, Depends(get_session)]
+PageParam = Annotated[int, Query(ge=1)]
+PageSizeParam = Annotated[int, Query(ge=1, le=100)]
+
+
+@router.get("/customers")
+async def list_customers(
+    session: SessionDep,
+    page: PageParam = 1,
+    page_size: PageSizeParam = 20,
+    channel: Annotated[str | None, Query()] = None,
+    department_id: Annotated[str | None, Query()] = None,
+    owner_account_id: Annotated[str | None, Query()] = None,
+) -> Page[CustomerOut]:
+    """List customers with optional filters and pagination."""
+    conditions = []
+    if channel is not None:
+        conditions.append(Customer.channel == channel)
+    if owner_account_id is not None:
+        conditions.append(Customer.owner_account_id == owner_account_id)
+    if department_id is not None:
+        # Customer has no dept column; resolve via the owning staff member.
+        dept_user_ids = sa.select(Staff.user_id).where(
+            Staff.dept_id == department_id
+        )
+        conditions.append(Customer.user_id.in_(dept_user_ids))
+
+    total_stmt = sa.select(sa.func.count()).select_from(Customer)
+    if conditions:
+        total_stmt = total_stmt.where(*conditions)
+    total = (await session.execute(total_stmt)).scalar_one()
+
+    list_stmt = sa.select(Customer)
+    if conditions:
+        list_stmt = list_stmt.where(*conditions)
+    list_stmt = (
+        list_stmt.order_by(
+            sa.nullslast(Customer.last_chat_time.desc()),
+            Customer.friend_id.asc(),
+        )
+        .limit(page_size)
+        .offset((page - 1) * page_size)
+    )
+
+    rows = (await session.execute(list_stmt)).scalars().all()
+    items = [CustomerOut.model_validate(row) for row in rows]
+    return Page(items=items, total=total, page=page, page_size=page_size)