feat(db): stock CHECK constraints + audit_logs DATETIME (2038 fix)

Migration 20260612234500: CHECK (quantity >= 0) on sklad_batches and
sklad_item_locations — the DB itself now rejects negative stock, so a
future C2-class bug (cumulative decrements driving a batch negative and
silently vanishing from totals) becomes a loud error instead of hidden
corruption. All three databases verified clean of negative rows before
the migration.

audit_logs.created_at TIMESTAMP(0) -> DATETIME(0): TIMESTAMP dies in 2038
and converts through the session timezone; existing wall times preserved
by the MODIFY.

Pinned by db-constraints.test.ts (4 tests, each watched fail pre-migration:
negative insert/update/location all succeeded before, 2098 audit timestamp
was rejected before).

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

prisma/migrations/20260612234500_stock_checks_and_audit_datetime/migration.sql

@@ -0,0 +1,14 @@
+-- AlterTable
+ALTER TABLE `audit_logs` MODIFY `created_at` DATETIME(0) NULL DEFAULT CURRENT_TIMESTAMP(0);
+
+
+-- Stock invariants enforced by the database itself: a future cumulative-
+-- decrement bug (the C2 class — duplicate issue lines driving a batch
+-- negative and silently vanishing from stock totals) becomes a loud error
+-- instead of hidden corruption. All databases verified clean of negative
+-- rows before this migration (2026-06-12).
+ALTER TABLE `sklad_batches`
+  ADD CONSTRAINT `chk_sklad_batches_quantity_nonneg` CHECK (`quantity` >= 0);
+
+ALTER TABLE `sklad_item_locations`
+  ADD CONSTRAINT `chk_sklad_item_locations_quantity_nonneg` CHECK (`quantity` >= 0);