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.

app/schemas/ticket.py

@@ -46,6 +46,7 @@ class TicketCreate(BaseModel):
     assigned_to: int | None = None
     customer_name: str | None = None
     phone: str | None = None
+    reported_at: datetime | None = None  # original report date for backdated/backfilled tickets; defaults to now when omitted
 
 
 class TicketUpdate(BaseModel):
@@ -100,6 +101,7 @@ class TicketBrief(BaseModel):
     description: str | None = None
     sla_deadline: datetime | None = None
     reopen_count: int = 0
+    reported_at: datetime | None = None
     created_at: datetime
     updated_at: datetime