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/schemas.py

@@ -0,0 +1,43 @@
+"""Pydantic v2 response models for the yunke data-mirror API."""
+
+from __future__ import annotations
+
+from typing import Generic, TypeVar
+
+from pydantic import BaseModel, ConfigDict
+
+T = TypeVar("T")
+
+
+class CustomerOut(BaseModel):
+    """Public Customer view — a subset of :class:`app.db.models.Customer`.
+
+    Only the fields the viewing frontend needs today are exposed; the verbatim
+    ``raw_json`` blob and internal audit columns stay server-side.
+    """
+
+    model_config = ConfigDict(from_attributes=True)
+
+    friend_id: str
+    channel: str
+    owner_account_id: str | None = None
+    friend_nickname: str | None = None
+    friend_remark: str | None = None
+    real_name: str | None = None
+    phone: str | None = None
+    region: str | None = None
+    sale_name: str | None = None
+    dept_name: str | None = None
+    head_image_url: str | None = None
+    last_chat_time: int | None = None
+    message_count: int | None = None
+    version: str | None = None
+
+
+class Page(BaseModel, Generic[T]):
+    """Generic pagination envelope returned by list endpoints."""
+
+    items: list[T]
+    total: int
+    page: int
+    page_size: int