feat: Wahab demo prep — Cancelled status, phone persistence, admin delete, users picker, detail-page fixes

- Add Cancelled as terminal status reachable from all active states; exclude
  from SLA breach reporting and dashboard active counts; add to status pickers
- Persist customer phone on ticket create/update (was silently dropped);
  add tickets.phone migration + legacy self-heal guard
- Add admin-only DELETE /api/tickets/{id} (removes timeline/photos/escalations)
- Add GET /api/auth/users for the assign-technician dropdown (was hardcoded)
- TicketOut now returns nested unit/category so the detail page stops showing
  '—' for Unit/Property/Category
- Ticket numbering uses max+1 so deletions never re-issue a number
- New-issue form: require Category and (standard mode) Priority client-side
- Tests: 11 new cases covering cancellation, SLA exemption, phone, delete,
  users endpoint, numbering

app/schemas/ticket.py

@@ -95,6 +95,7 @@ class TicketBrief(BaseModel):
     assigned_to: int | None = None
     assigned_technician_name: str | None = None
     reporter: str | None = None
+    phone: str | None = None
     description: str | None = None
     sla_deadline: datetime | None = None
     reopen_count: int = 0
@@ -113,6 +114,8 @@ class TicketOut(TicketBrief):
     customer_rating: int | None = None
     timeline: list[TicketTimelineOut] = []
     photos: list[TicketPhotoOut] = []
+    unit: UnitOut | None = None
+    category: CategoryOut | None = None
     sla_status: dict | None = None