fix: harden WebUI bugfix sweep

api/models.py

@@ -594,6 +594,8 @@ class Session:
             self.truncation_watermark = None
 
     def save(self, touch_updated_at: bool = True, skip_index: bool = False) -> None:
+        if not is_safe_session_id(self.session_id):
+            raise ValueError(f"Unsafe session_id {self.session_id!r}; refusing to write outside session store")
         # ── #1558 P0 guard ──────────────────────────────────────────────
         # Refuse to save a session that was loaded with metadata_only=True.
         # Such sessions have messages=[] (it's the whole point of the partial