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

pyproject.toml

@@ -0,0 +1,50 @@
+[project]
+name = "fastapi-template"
+version = "0.1.0"
+description = "开箱即用的 FastAPI 模板：MySQL(SQLAlchemy2.0 async) / MongoDB(Beanie) 可切换"
+requires-python = ">=3.11"
+dependencies = [
+    "fastapi>=0.115",
+    "uvicorn[standard]>=0.30",
+    "pydantic>=2.7",
+    "pydantic-settings>=2.4",
+    # MySQL backend
+    "sqlalchemy[asyncio]>=2.0",
+    "aiomysql>=0.2.0",
+    "alembic>=1.13",
+    # MongoDB backend
+    "beanie>=2.0",
+    "pymongo>=4.9",
+    "cryptography>=50.0.0",
+]
+
+[dependency-groups]
+dev = [
+    "pytest>=8.0",
+    "pytest-asyncio>=0.23",
+    "httpx>=0.27",
+    "ruff>=0.6",
+]
+
+[tool.ruff]
+line-length = 100
+target-version = "py311"
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "UP", "B", "ASYNC", "RUF"]
+ignore = [
+    "RUF001", "RUF002", "RUF003",  # 中文文档字符串里的全角标点是有意的
+]
+
+[tool.ruff.lint.per-file-ignores]
+"alembic/env.py" = ["E402", "F401"]   # 需要先 fileConfig 再 import 应用模块
+"alembic/versions/*" = ["I001"]       # 迁移文件是自动生成物，不整理 import
+"tests/conftest.py" = ["E402"]        # 需要先设环境变量再 import app
+
+[tool.pytest.ini_options]
+asyncio_mode = "auto"
+testpaths = ["tests"]
+pythonpath = ["."]
+
+[tool.uv]
+package = false