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/042_equipment_no_overlap.sql

@@ -0,0 +1,23 @@
+-- =============================================================================
+-- 042: Equipment reservation overlap — database-enforced (P0 fix)
+-- =============================================================================
+-- reserveEquipment() did a check-then-insert with no DB guard: two concurrent
+-- requests both pass the in-memory overlap check and both insert, double-
+-- booking a physical machine. A GiST exclusion constraint makes the database
+-- the arbiter. Only 'reserved' rows conflict (cancelled/completed do not).
+-- btree_gist provides the `uuid WITH =` operator class for the composite.
+--
+-- Idempotent: DROP CONSTRAINT IF EXISTS then ADD (ADD CONSTRAINT has no
+-- IF NOT EXISTS form).
+-- =============================================================================
+
+CREATE EXTENSION IF NOT EXISTS btree_gist;
+
+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');