Sanitize user-visible employee sync failures

[Copilot]

Alert #13 flagged that raw exception text from employee sync failures could be persisted in sync status and surfaced through the settings API/UI. This change limits externally visible failure details to safe messages while preserving server-side diagnostics.

• What changed

  • Replaced persisted str(exc) values in employee sync failure handling with a fixed generic message for unexpected errors.
  • Kept logging unchanged so full exception details remain available in server logs.

• Behavioral adjustment

  • Preserved the explicit "No employees returned from Graph API." status as a safe, intentional user-facing failure case instead of routing it through generic exception handling.

• Tests

  • Updated focused sync-status coverage to assert the sanitized error message for unexpected failures.

except Exception as exc:
    logger.exception("Employee data sync failed (source=%s): %s", source, exc)
    mark_data_update_finished(
        success=False,
        error="Employee sync failed due to an internal error.",
        source=source,
    )