fix: preserve takeover and share token integrity

digital-avatar-app/backend/models.py

@@ -20,7 +20,7 @@ class Avatar(Base):
     photo_url = Column(String, default="")
     emoji = Column(String, default="🤖")
     status = Column(String, default="active")  # active | inactive | training
-    share_token = Column(String, default="", unique=True, index=True)  # 对外分享使用的不可猜测令牌
+    share_token = Column(String, nullable=True, default=None, unique=True, index=True)  # 对外分享使用的不可猜测令牌
     token_balance = Column(Integer, default=0)
     config = Column(JSON, default=dict)
     created_at = Column(DateTime, server_default=func.now())
@@ -36,7 +36,7 @@ class Avatar(Base):
             "photoUrl": self.photo_url,
             "emoji": self.emoji,
             "status": self.status,
-            "shareToken": self.share_token,
+            "shareToken": self.share_token or "",
             "tokenBalance": self.token_balance,
             "config": self.config or {},
             "createdAt": _iso(self.created_at),