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/sync_integration_test.go

@@ -18,7 +18,11 @@ func setupSyncTestDB(t *testing.T) *database.Queries {
 	ctx := context.Background()
 
 	dbURL := "postgresql://postgres:postgres@db:5432/bookhoard?sslmode=disable"
-	dbPool, err := pgxpool.New(ctx, dbURL)
+	// Use max_conns=1 to prevent connection pool exhaustion during test runs
+	dbConfig, err := pgxpool.ParseConfig(dbURL)
+	require.NoError(t, err, "Failed to parse database URL")
+	dbConfig.MaxConns = 1
+	dbPool, err := pgxpool.NewWithConfig(ctx, dbConfig)
 	require.NoError(t, err, "Failed to connect to test database")
 
 	db := database.New(dbPool)