Fix: exclude cancelled bookings from the no-overlap constraint

bookings_no_overlap (the GiST exclusion constraint preventing
double-booking) had no WHERE clause, so a cancelled booking's old time
range stayed "occupied" forever -- permanently blocking that
resource+slot from ever being booked again, even though the booking
itself is dead. Found live: a cancelled test booking blocked a real
reschedule attempt into the same slot ("overlaps with an existing
appointment" even though nothing active was there).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backoffice/db/init.sql

@@ -166,16 +166,23 @@ ALTER TABLE bookings ADD COLUMN IF NOT EXISTS resource_id text;
 ALTER TABLE bookings ADD COLUMN IF NOT EXISTS during tstzrange
     GENERATED ALWAYS AS (tstzrange(start_time, end_time, '[)')) STORED;
 
--- ALTER TABLE ... ADD CONSTRAINT has no IF NOT EXISTS form, so guard by name
--- to keep this file safe to re-run on every deploy like everything above it.
+-- ALTER TABLE ... ADD CONSTRAINT has no IF NOT EXISTS form, so drop-then-add
+-- unconditionally to keep this file safe to re-run on every deploy like
+-- everything above it. The WHERE clause is load-bearing: without it, a
+-- cancelled booking's old time range stays "occupied" forever, permanently
+-- blocking that resource+slot from ever being booked again even though the
+-- booking itself is dead -- found 2026-09-12 when a cancelled test booking
+-- blocked a real reschedule into the same slot.
 DO $$
 BEGIN
-  IF NOT EXISTS (
+  IF EXISTS (
     SELECT 1 FROM pg_constraint WHERE conname = 'bookings_no_overlap'
   ) THEN
-    ALTER TABLE bookings ADD CONSTRAINT bookings_no_overlap
-      EXCLUDE USING gist (resource_id WITH =, during WITH &&);
+    ALTER TABLE bookings DROP CONSTRAINT bookings_no_overlap;
   END IF;
+  ALTER TABLE bookings ADD CONSTRAINT bookings_no_overlap
+    EXCLUDE USING gist (resource_id WITH =, during WITH &&)
+    WHERE (status <> 'cancelled');
 END $$;
 
 ALTER TABLE clients ADD COLUMN IF NOT EXISTS slug         text UNIQUE;