fix: refactor conversation pagination to use SQLAlchemy session manag… (#11956)

Signed-off-by: -LAN- <laipz8200@outlook.com>

api/models/web.py

@@ -1,4 +1,5 @@
 from sqlalchemy import func
+from sqlalchemy.orm import Mapped, mapped_column
 
 from .engine import db
 from .model import Message
@@ -33,7 +34,7 @@ class PinnedConversation(db.Model):
 
     id = db.Column(StringUUID, server_default=db.text("uuid_generate_v4()"))
     app_id = db.Column(StringUUID, nullable=False)
-    conversation_id = db.Column(StringUUID, nullable=False)
+    conversation_id: Mapped[str] = mapped_column(StringUUID)
     created_by_role = db.Column(db.String(255), nullable=False, server_default=db.text("'end_user'::character varying"))
     created_by = db.Column(StringUUID, nullable=False)
     created_at = db.Column(db.DateTime, nullable=False, server_default=func.current_timestamp())