fix: use UUID as ticket id instead of ticketNumber

ticketNumber (e.g. TK-2026-296691) was being used as id, causing
GET /api/tickets/{id} to 404 on upstream Gongdan API which expects UUID.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/tickets.py

@@ -42,7 +42,8 @@ def _map_priority(raw: str) -> str:
 def _transform_ticket(t: dict) -> dict:
     """Transform a Gongdan ticket to the frontend TicketData shape."""
     return {
-        "id": t.get("ticketNumber", t.get("id", "")),
+        "id": t.get("id", ""),
+        "ticketNumber": t.get("ticketNumber", ""),
         "title": t.get("description", "")[:120] or "No description",
         "status": _map_status(t.get("status", "")),
         "priority": _map_priority(t.get("priority", "")),