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

alembic/versions/81076765be75_init_items_table.py

@@ -0,0 +1,36 @@
+"""init items table
+
+Revision ID: 81076765be75
+Revises: 
+Create Date: 2026-08-21 21:21:49.098436
+"""
+from collections.abc import Sequence
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = '81076765be75'
+down_revision: str | None = None
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('items',
+    sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
+    sa.Column('name', sa.String(length=128), nullable=False),
+    sa.Column('description', sa.Text(), nullable=True),
+    sa.Column('created_at', sa.DateTime(), server_default=sa.text('now()'), nullable=False),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_items_name'), 'items', ['name'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_items_name'), table_name='items')
+    op.drop_table('items')
+    # ### end Alembic commands ###

alembic/versions/README.md

@@ -0,0 +1,3 @@
+# 迁移版本文件目录。
+# 生成迁移：uv run alembic revision --autogenerate -m "add xxx table"
+# 执行迁移：uv run alembic upgrade head