test(infrastructure): Configure pgxpool with max_conns=1

Fix database connection exhaustion in tests by setting max_conns=1
when creating pgxpool via pgxpool.ParseConfig().

- Update setupTestServer() in test_helpers.go
- Update setupSyncTestDB() in sync_integration_test.go

This reduces per-test connection usage from 4 to 1, keeping total
connections well under PostgreSQL's default max_connections=100.

78 tests × 1 connection = 78 connections (down from 312 potential)

Fixes test failures: "FATAL: sorry, too many clients already"

See PROJECT_GUIDELINES.md Testing section for details.

cmd/server/tests/test_helpers.go

@@ -309,7 +309,12 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 	cfg.RequestsPerMinute = 1000
 
 	// Connect to test database using the same method as main application
-	dbPool, err := pgxpool.New(context.Background(), cfg.DatabaseURL())
+	// Use max_conns=1 to prevent connection pool exhaustion during test runs
+	// (78 tests × 1 connection = 78 connections, well under PostgreSQL's 100 default max_connections)
+	dbConfig, err := pgxpool.ParseConfig(cfg.DatabaseURL())
+	require.NoError(t, err, "Failed to parse database URL")
+	dbConfig.MaxConns = 1
+	dbPool, err := pgxpool.NewWithConfig(context.Background(), dbConfig)
 	require.NoError(t, err, "Failed to connect to test database")
 
 	queries := database.New(dbPool)