feat: allow bookings.status to be marked completed

supabase/migrations/20260812105003_add_completed_booking_status.sql

@@ -0,0 +1,5 @@
+-- Widen the bookings.status check constraint to allow 'completed', so the
+-- admin panel can mark past appointments completed (not just cancelled).
+alter table public.bookings drop constraint bookings_status_check;
+alter table public.bookings add constraint bookings_status_check
+  check (status in ('confirmed', 'cancelled', 'completed'));