fix(12): resolve critical code review findings (CR-01 through CR-04)

- CR-01: fix browse URL from /folders/{id} to /items?parent_ref= (broken feature)
- CR-02: remove raw provider exception text from _TerminalProviderError messages
- CR-03: add user_id predicate to get_or_create_folder_state query (ownership boundary)
- CR-04: add max_length=255 to ConnectionRenameRequest.display_name

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tasks/cloud_tasks.py

@@ -107,7 +107,7 @@ async def _run(user_id: str, connection_id: str, parent_ref: str | None) -> dict
                 error_message="Credential decryption failed. Re-connect the account.",
             )
             await session.commit()
-            raise _TerminalProviderError(f"Credential decryption failed: {exc}") from exc
+            raise _TerminalProviderError("Credential decryption failed. Re-connect the account.") from exc
 
         # Build adapter and fetch listing
         try:
@@ -133,9 +133,9 @@ async def _run(user_id: str, connection_id: str, parent_ref: str | None) -> dict
                     error_message="Authentication failed. Re-connect the account.",
                 )
                 await session.commit()
-                raise _TerminalProviderError(f"Auth error: {exc}") from exc
+                raise _TerminalProviderError("Authentication failed. Re-connect the account.") from exc
             # Transient failure — let Celery retry
-            raise _TransientProviderError(f"Provider error: {exc}") from exc
+            raise _TransientProviderError("Provider error. Will retry.") from exc
 
         # Reconcile listing into durable cloud_items rows
         await reconcile_cloud_listing(