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/services/sla.py

@@ -55,9 +55,12 @@ def should_escalate_on_response(ticket: Ticket) -> bool:
     return datetime.now(timezone.utc) > deadline
 
 
+TERMINAL_STATUSES = {"Closed", "Completed", "Cancelled"}
+
+
 def is_sla_breached(ticket: Ticket) -> bool:
     """Return True if the ticket's resolution SLA deadline has passed."""
-    if ticket.sla_deadline is None or ticket.status in ("Closed", "Completed"):
+    if ticket.sla_deadline is None or ticket.status in TERMINAL_STATUSES:
         return False
     deadline = ticket.sla_deadline
     if deadline.tzinfo is None:
@@ -88,7 +91,7 @@ async def get_sla_status(ticket: Ticket) -> dict:
         rd = resolution_deadline
         if rd.tzinfo is None:
             rd = rd.replace(tzinfo=timezone.utc)
-        resolution_breached = now > rd if ticket.status not in ("Closed", "Completed") else False
+        resolution_breached = now > rd if ticket.status not in TERMINAL_STATUSES else False
 
     return {
         "priority": ticket.priority,