feat: ship persistent ai playground and alerts foundation

backend/app/models/playground_session.py

@@ -0,0 +1,27 @@
+from sqlalchemy import JSON, Column, DateTime, ForeignKey, Integer, String, UniqueConstraint
+from sqlalchemy.sql import func
+
+from app.db.session import Base
+
+
+class PlaygroundSession(Base):
+    __tablename__ = "playground_sessions"
+    __table_args__ = (
+        UniqueConstraint("user_id", "session_key", name="uq_playground_sessions_user_session_key"),
+    )
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    user_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    session_key = Column(String(100), nullable=False, default="default")
+    title = Column(String(200), nullable=False, default="Playground 会话")
+    state = Column(JSON, nullable=False, default={})
+    created_at = Column(DateTime(timezone=True), server_default=func.now(), nullable=False)
+    updated_at = Column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False,
+    )
+
+    def __repr__(self):
+        return f"<PlaygroundSession user_id={self.user_id} session_key={self.session_key}>"