fix(equipment): DB-enforced reservation no-overlap (D1, P0)

reserveEquipment was check-then-insert with no DB guard: two concurrent
requests both passed the in-memory overlap check and double-booked a
machine. Migration 042 adds btree_gist + a GiST EXCLUDE constraint
(equipment_id =, tstzrange && ) WHERE status='reserved', making the DB
the arbiter; the action maps the 23P01 violation to the existing
friendly overlap message. App-side check kept as a fast pre-check.
Schema + DB maps updated.

scripts/schema.sql

@@ -20,6 +20,9 @@
 -- 1. EXTENSIONS
 -- -----------------------------------------------------------------------------
 CREATE EXTENSION IF NOT EXISTS "uuid-ossp";
+-- btree_gist: provides `uuid WITH =` for the equipment_reservations overlap
+-- exclusion constraint (see section below / scripts/042).
+CREATE EXTENSION IF NOT EXISTS btree_gist;
 
 
 -- -----------------------------------------------------------------------------
@@ -2277,6 +2280,18 @@ CREATE INDEX IF NOT EXISTS idx_equipment_res_space     ON public.equipment_reser
 CREATE INDEX IF NOT EXISTS idx_equipment_res_equipment ON public.equipment_reservations (equipment_id, starts_at);
 CREATE INDEX IF NOT EXISTS idx_equipment_res_member    ON public.equipment_reservations (member_id);
 
+-- DB-enforced no-overlap (equivalent to scripts/042). Only 'reserved' rows
+-- conflict; the app-side check stays as a fast pre-check, the DB is the
+-- arbiter under concurrency. Idempotent: DROP then ADD.
+ALTER TABLE public.equipment_reservations
+  DROP CONSTRAINT IF EXISTS equipment_reservations_no_overlap;
+ALTER TABLE public.equipment_reservations
+  ADD CONSTRAINT equipment_reservations_no_overlap
+  EXCLUDE USING gist (
+    equipment_id WITH =,
+    tstzrange(starts_at, ends_at, '[)') WITH &&
+  ) WHERE (status = 'reserved');
+
 ALTER TABLE public.equipment              ENABLE ROW LEVEL SECURITY;
 ALTER TABLE public.equipment_reservations ENABLE ROW LEVEL SECURITY;