feat: 改造为开箱即用模板 — MySQL/MongoDB(Beanie) 一键切换 + AGENTS.md 开发规范

- src/ 按域组织骨架，示例域 items 双后端实现（SQLAlchemy2.0 async / Beanie 2.x）
- DB_BACKEND 环境变量切换，router 只依赖 Repo Protocol
- alembic 迁移（含初始 items 迁移）、uv 依赖管理、ruff、pytest 异步测试
- Dockerfile + docker-compose（app/mysql/mongo）
- AGENTS.md 重写为本模板开发规范；原最佳实践文档归档 docs/
- 验证：mongodb/mysql 双后端 pytest 全绿 + uvicorn 真实冒烟通过

tests/test_items.py

@@ -0,0 +1,33 @@
+async def test_health(client):
+    resp = await client.get("/health")
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "ok"
+
+
+async def test_items_crud(client):
+    # create
+    resp = await client.post("/api/v1/items", json={"name": "demo", "description": "d"})
+    assert resp.status_code == 201
+    item = resp.json()
+    assert item["name"] == "demo"
+    item_id = item["id"]
+
+    # get
+    resp = await client.get(f"/api/v1/items/{item_id}")
+    assert resp.status_code == 200
+
+    # list
+    resp = await client.get("/api/v1/items")
+    assert resp.status_code == 200
+    assert resp.json()["total"] >= 1
+
+    # update
+    resp = await client.patch(f"/api/v1/items/{item_id}", json={"name": "demo2"})
+    assert resp.status_code == 200
+    assert resp.json()["name"] == "demo2"
+
+    # delete
+    resp = await client.delete(f"/api/v1/items/{item_id}")
+    assert resp.status_code == 204
+    resp = await client.get(f"/api/v1/items/{item_id}")
+    assert resp.status_code == 404