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/schema.prisma

@@ -65,7 +65,9 @@ model audit_logs {
   new_values  String?   @db.LongText
   user_agent  String?   @db.Text
   session_id  String?   @db.VarChar(128)
-  created_at  DateTime? @default(now()) @db.Timestamp(0)
+  // DATETIME, not TIMESTAMP — TIMESTAMP dies in 2038 and converts through
+  // the session timezone (2026-06-12 hardening migration).
+  created_at  DateTime? @default(now()) @db.DateTime(0)
 
   @@index([action], map: "idx_audit_logs_action")
   @@index([created_at], map: "idx_audit_logs_created")