fix(testdb): fallback to POSTGRES_HOST in dbHost()

dbHost() only checked TEST_DB_HOST, defaulting to localhost. When the
CI workflow removed PG service port mappings (to avoid host port conflicts),
all tests using testdb.CreateTestDatabase() failed because localhost:5432
was unreachable inside the job container.

The workflow sets POSTGRES_HOST=postgres for the db package's Connect(),
but testdb's dbHost() never checked this env var. Add POSTGRES_HOST as
a second fallback so all packages using testdb automatically work with
the existing CI configuration.

backend/testutils/testdb/testdb.go

@@ -33,6 +33,9 @@ func dbHost() string {
 	if h := os.Getenv("TEST_DB_HOST"); h != "" {
 		return h
 	}
+	if h := os.Getenv("POSTGRES_HOST"); h != "" {
+		return h
+	}
 	return "localhost"
 }