fix(backend): harden final MVP invariants

backend/internal/logic/ai/rate_limit_postgres_test.go

@@ -1,3 +1,5 @@
+//go:build integration
+
 package ai
 
 import (
@@ -16,10 +18,8 @@ import (
 )
 
 func TestPostgresReserveRateLimitIsAtomicAcrossConcurrentConnections(t *testing.T) {
-	dsn := os.Getenv("DATABASE_URL")
-	if dsn == "" {
-		t.Skip("DATABASE_URL is not configured; skipping PostgreSQL AI rate reservation test")
-	}
+	dsn := os.Getenv("TEST_DATABASE_URL")
+	require.NotEmpty(t, dsn, "TEST_DATABASE_URL is required for integration tests and must point to an isolated database")
 	database, err := gorm.Open(postgres.Open(dsn), &gorm.Config{TranslateError: true, Logger: logger.Default.LogMode(logger.Silent)})
 	require.NoError(t, err)
 	require.NoError(t, models.AutoMigrate(database))