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/models/ticket.py

@@ -55,6 +55,11 @@ class Ticket(Base):
     customer_rating: Mapped[int | None] = mapped_column(Integer, nullable=True)
     reopen_count: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
     closed_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
+    reported_at: Mapped[datetime | None] = mapped_column(
+        DateTime,
+        nullable=True,
+        comment="Original reported date. Backdated/backfilled tickets keep their true report date; NULL falls back to created_at.",
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now(), nullable=False)
     updated_at: Mapped[datetime] = mapped_column(
         DateTime,