feat: backdated reported date for old active tickets

Wahab can now enter historical tickets that keep their true reported date:

- Add nullable tickets.reported_at (DateTime) via alembic d5e0f2a1c3b4;
  backfill existing rows from created_at so nothing shows empty.
- TicketCreate.reported_at (optional) is persisted by create_ticket and
  defaults to now when omitted, so existing create behavior is unchanged.
- New-issue form gains a 'Reported Date' date picker (defaults to today,
  past dates allowed, future blocked) and sends reported_at in the payload.
- List and detail pages show 'Reported' next to 'Created' (date-only,
  labeled) so backdated tickets are obvious; SLA deadline is unchanged and
  still runs from creation time so backfilling never instantly breaches.
- Tests: backdated create persists + is exposed on list/detail; omitted
  reported_at defaults to now; SLA window unchanged; full datetime round trip.

AGENTS.md

@@ -94,6 +94,11 @@ Frontend: Alpine.js (CDN) + Tailwind CSS (CDN). Auth state in localStorage. Role
 
 ## Ticket System (Sprint 2)
 
+- `reported_at` (nullable DateTime, alembic `d5e0f2a1c3b4`) records a ticket's original reported date;
+  `TicketCreate.reported_at` lets Admin/Wahab enter backdated tickets that stay active. It defaults to
+  now when omitted (migration backfilled existing rows from `created_at`). SLA deadlines run from
+  `created_at`, not `reported_at` — backfilling history never instantly breaches a ticket.
+
 ### Status Lifecycle (16 statuses)
 New → Logged → Triage → Assigned → Accepted → Travelling → On Site → In Progress → Waiting Parts → Escalated → Completed → On-Field Verification → Wahab Review → Closed → Reopened → Cancelled