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/api/user/sources/upload.py

@@ -24,6 +24,7 @@ from docsgpt.security.zip_archive import (
     ZipExtractionError,
     ZipExtractionLimits,
 )
+from docsgpt.storage.db.repositories.connector_sessions import ConnectorSessionsRepository
 from docsgpt.storage.db.repositories.idempotency import IdempotencyRepository
 from docsgpt.storage.db.repositories.sources import SourcesRepository
 from docsgpt.storage.db.source_config import SourceConfig
@@ -496,6 +497,15 @@ class UploadRemote(Resource):
                         ),
                         400,
                     )
+                with db_readonly() as conn:
+                    connector_session = ConnectorSessionsRepository(conn).get_by_session_token(session_token)
+                if not connector_session or connector_session.get("user_id") != user:
+                    if scoped_key:
+                        _release_claim(scoped_key)
+                    return make_response(
+                        jsonify({"success": False, "error": "Invalid or unauthorized session"}),
+                        401,
+                    )
                 # Process file_ids
 
                 file_ids = config.get("file_ids", [])