feat: Enhance reservation functionality

supabase/migrations/20260421000001_overlap_constraint_exclude_cancelled.sql

@@ -0,0 +1,12 @@
+-- The overlap exclusion constraint must not apply to cancelled reservations,
+-- so that a cancelled slot can be re-booked by another member.
+
+alter table public.reservations
+  drop constraint no_overlapping_reservations;
+
+alter table public.reservations
+  add constraint no_overlapping_reservations
+  exclude using gist (
+    boat_id with =,
+    tstzrange(start_time, end_time, '[)') with &&
+  ) where (status <> 'cancelled');