✨ feat(user): foundation for parallel-safe BDD isolation (T12 stage 1/2) (#34)

NewPostgresRepositoryFromDSN factory + BuildSchemaIsolatedDSN helper + integration test proving per-schema isolation works at repo level. Foundation for T12. Wiring into testserver is stage 2/2.

Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

pkg/user/postgres_repository.go

@@ -125,6 +125,67 @@ func NewPostgresRepository(cfg *config.Config) (*PostgresRepository, error) {
 	return repo, nil
 }
 
+// NewPostgresRepositoryFromDSN creates a PostgresRepository connected via the given DSN
+// and runs AutoMigrate against it. Used by BDD test infra to create a per-scenario
+// repository pointing at an isolated schema (the DSN typically includes search_path=<schema>).
+//
+// Pass the same cfg used elsewhere (it is required by methods that read pool settings),
+// but the DSN passed here OVERRIDES the host/port/dbname/etc that cfg would have built.
+func NewPostgresRepositoryFromDSN(cfg *config.Config, dsn string) (*PostgresRepository, error) {
+	repo := &PostgresRepository{
+		config:     cfg,
+		spanPrefix: "user.repo.",
+	}
+
+	gormLogger := logger.New(
+		log.New(os.Stderr, "\n", log.LstdFlags),
+		logger.Config{
+			SlowThreshold:             time.Second,
+			LogLevel:                  logger.Warn,
+			IgnoreRecordNotFoundError: true,
+			Colorful:                  false,
+		},
+	)
+
+	db, err := gorm.Open(postgres.Open(dsn), &gorm.Config{Logger: gormLogger})
+	if err != nil {
+		return nil, fmt.Errorf("failed to connect to PostgreSQL with custom DSN: %w", err)
+	}
+
+	sqlDB, err := db.DB()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get sql.DB from gorm: %w", err)
+	}
+	sqlDB.SetMaxOpenConns(cfg.GetDatabaseMaxOpenConns())
+	sqlDB.SetMaxIdleConns(cfg.GetDatabaseMaxIdleConns())
+	sqlDB.SetConnMaxLifetime(cfg.GetDatabaseConnMaxLifetime())
+
+	if err := db.AutoMigrate(&User{}); err != nil {
+		return nil, fmt.Errorf("failed to auto-migrate via custom DSN: %w", err)
+	}
+
+	repo.db = db
+	return repo, nil
+}
+
+// BuildSchemaIsolatedDSN returns a Postgres DSN that targets the given schema via
+// the search_path connection parameter. Use with NewPostgresRepositoryFromDSN to
+// get a repository whose connection only sees the per-scenario schema.
+func BuildSchemaIsolatedDSN(cfg *config.Config, schemaName string) string {
+	return fmt.Sprintf(
+		"host=%s port=%d user=%s password=%s dbname=%s sslmode=%s search_path=%s",
+		cfg.GetDatabaseHost(),
+		cfg.GetDatabasePort(),
+		cfg.GetDatabaseUser(),
+		cfg.GetDatabasePassword(),
+		cfg.GetDatabaseName(),
+		cfg.GetDatabaseSSLMode(),
+		schemaName,
+	)
+}
+
+// (Close already exists below; we reuse it.)
+
 // initializeDatabase sets up the PostgreSQL database connection and runs migrations
 func (r *PostgresRepository) initializeDatabase() error {
 	// Configure GORM logger based on config