fix(backend): use UUID type for ticket_id parameter in get_ticket handler

Litestar's {ticket_id:uuid} path parameter requires the handler
parameter to be typed as UUID, not str. This caused a 400 validation
error on GET /api/tickets/{uuid}.

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

backend/app/api/tickets.py

@@ -7,6 +7,7 @@ Returns data in a format aligned with the frontend TicketData interface:
 from __future__ import annotations
 
 from collections import Counter
+from uuid import UUID
 
 import httpx
 from litestar import get
@@ -116,7 +117,7 @@ async def list_tickets(page: int = 1, page_size: int = 20) -> list[dict]:
 
 
 @get("/api/tickets/{ticket_id:uuid}")
-async def get_ticket(ticket_id: str) -> dict:
+async def get_ticket(ticket_id: UUID) -> dict:
     """GET /api/tickets/:id — Get a single ticket detail."""
     url = f"{settings.gongdan_api_base}/api/tickets/{ticket_id}"