fix: gracefully handle missing avatar SQLite database

- Return empty list instead of crashing when avatar.db doesn't exist
- Add is_available() check before each endpoint
- 503 status when trying to access unavailable database

backend/app/services/avatar_service.py

@@ -23,7 +23,8 @@ def _get_engine_and_session():
         if not os.path.isabs(db_path):
             db_path = os.path.abspath(db_path)
         if not os.path.exists(db_path):
-            raise FileNotFoundError(f"数字分身数据库不存在: {db_path}")
+            # 数据库不存在时返回 None，由调用方处理
+            return None, None
         _engine = create_engine(
             f"sqlite:///{db_path}",
             connect_args={"check_same_thread": False},
@@ -32,11 +33,17 @@ def _get_engine_and_session():
     return _engine, _SessionLocal()
 
 
-def get_session() -> Session:
+def get_session() -> Optional[Session]:
     _, session = _get_engine_and_session()
     return session
 
 
+def is_available() -> bool:
+    """检查数字分身数据库是否可用"""
+    engine, _ = _get_engine_and_session()
+    return engine is not None
+
+
 class AvatarService:
 
     @staticmethod