chore: initialize medical consultation agent demo

backend/app/db/__init__.py

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+"""数据库连接、声明式基类和会话依赖。"""

backend/app/db/base.py

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+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    """SQLAlchemy 基类：所有 ORM 模型统一继承该基类。"""
+
+    pass

backend/app/db/session.py

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+from collections.abc import Generator
+from pathlib import Path
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session, sessionmaker
+
+from app.core.config import settings
+
+
+def _engine_kwargs() -> dict:
+    """数据库连接：根据数据库类型设置 SQLAlchemy engine 参数。"""
+    if settings.database_url.startswith("sqlite"):
+        Path("storage").mkdir(exist_ok=True)
+        return {"connect_args": {"check_same_thread": False}}
+    return {"pool_pre_ping": True, "pool_recycle": 3600}
+
+
+engine = create_engine(settings.database_url, **_engine_kwargs())
+SessionLocal = sessionmaker(bind=engine, autoflush=False, autocommit=False, expire_on_commit=False)
+
+
+def get_db() -> Generator[Session, None, None]:
+    """请求依赖：为每个请求创建数据库会话并在结束后关闭。"""
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()