重组为 backend/clients/docs 三层结构，并清理 git 污染。

将后端迁入 backend/，完善根目录 .gitignore，删除误提交的 .mypy_cache 缓存文件。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/conftest.py

@@ -0,0 +1,28 @@
+"""Shared test fixtures (SQLite memory DB session factory)."""
+
+from __future__ import annotations
+
+from collections.abc import AsyncGenerator
+
+import pytest_asyncio
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+import app.db.models  # noqa: F401  # register ORM tables on Base.metadata
+from app.db.base import Base
+
+
+@pytest_asyncio.fixture
+async def sqlite_session_factory() -> AsyncGenerator[async_sessionmaker[AsyncSession], None]:
+    """In-memory SQLite + create_all; yields async_sessionmaker suitable for injection."""
+    engine = create_async_engine("sqlite+aiosqlite:///:memory:")
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+    factory = async_sessionmaker(
+        engine,
+        class_=AsyncSession,
+        expire_on_commit=False,
+        autoflush=False,
+        autobegin=False,
+    )
+    yield factory
+    await engine.dispose()