feat: FastAPI skeleton with config, database, health endpoint

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/database.py

@@ -0,0 +1,17 @@
+"""异步 SQLAlchemy 引擎 + session 工厂。"""
+from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker, AsyncSession
+from sqlalchemy.orm import DeclarativeBase
+
+from app.config import settings
+
+engine = create_async_engine(settings.database_url, echo=False)
+async_session = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+async def get_db() -> AsyncSession:
+    async with async_session() as session:
+        yield session