fix: stop leaking connector OAuth session tokens to other origins

The connector OAuth popup posted the session token to window.opener with
a '*' target origin, so any page that opened the popup received it. An
attacker with an account on a multi-user deployment could start a flow for
their own pending session, get a victim to finish the provider consent, and
receive a token backed by the victim's Drive/SharePoint/Confluence tokens.

- Post popup results only to allowed frontend origins: the callback origin,
  OIDC_FRONTEND_URL, the new CONNECTOR_ALLOWED_ORIGINS, and localhost:5173
  when the callback runs on a loopback host.
- Render the success page from the callback itself so the token never
  appears in a URL; callback-status ignores session_token/user_email params.
- ConnectorAuth accepts messages only from the popup it opened, on the
  callback origin reported by /api/connectors/auth.
- /api/connectors/disconnect requires auth and only deletes the caller's
  session.
- /api/connectors/sync and /api/remote reject session tokens the caller
  does not own.

Fixes #2766

docsgpt/storage/db/repositories/connector_sessions.py

@@ -307,11 +307,13 @@ class ConnectorSessionsRepository:
         result = self._conn.execute(text(sql), params)
         return result.rowcount > 0
 
-    def delete_by_session_token(self, session_token: str) -> bool:
+    def delete_by_session_token(self, session_token: str, user_id: str) -> bool:
+        """Delete the session behind ``session_token`` only if ``user_id`` owns it."""
         result = self._conn.execute(
             text(
-                "DELETE FROM connector_sessions WHERE session_token = :token"
+                "DELETE FROM connector_sessions "
+                "WHERE session_token = :token AND user_id = :user_id"
             ),
-            {"token": session_token},
+            {"token": session_token, "user_id": user_id},
         )
         return result.rowcount > 0