✨ feat(user): NewPostgresRepositoryFromDSN factory + integration test (T12 stage 1/2)

First building block for parallel-safe BDD scenario isolation (T12 plan,
ADR-0025 follow-up). PR #28 had to revert BDD_SCHEMA_ISOLATION because
SetupScenarioSchema created an empty schema without migrations -- the
production server's repo never saw it. This PR adds the missing piece:
a factory that opens a *PostgresRepository connected via an arbitrary
DSN AND runs AutoMigrate against it, so a per-scenario schema actually
gets the users table.

Public API additions in pkg/user/postgres_repository.go:

- NewPostgresRepositoryFromDSN(cfg, dsn) (*PostgresRepository, error)
  Opens the repo from an explicit DSN (overrides cfg's host/port/etc),
  runs AutoMigrate -- creates tables in whatever schema the DSN's
  search_path points to.

- BuildSchemaIsolatedDSN(cfg, schemaName) string
  Builds a DSN with `search_path=<schemaName>` from a base config.

The existing NewPostgresRepository(cfg) is unchanged. Existing Close()
method is reused.

Integration test in postgres_repository_isolated_test.go proves:
- AutoMigrate creates `users` table in the per-scenario schema (not public)
- A CreateUser through the isolated repo writes into the per-scenario schema
- public.users sees ZERO rows for the test username
- The per-scenario schema users table sees exactly 1 row

Test skips gracefully when DLC_DATABASE_HOST is not set.

Out of scope (T12 stage 2/2 next):
- Wiring this factory into pkg/bdd/testserver/SetupScenarioSchema
- Spawning a fresh server.Server per scenario (requires NewServerWithUserRepo)
- Removing -p 1 from scripts/run-bdd-tests.sh after parallel safety is achieved

Per code-reviewer skill SOLID/DDD section :
- SRP : factory has single responsibility (open + migrate, no business logic)
- OCP : the new factory extends the package without changing existing callers
- Cognitive load : 1 file, 50 lines added, 1 dedicated test file

🤖 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.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

pkg/user/postgres_repository_isolated_test.go

@@ -0,0 +1,118 @@
+package user
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"testing"
+
+	"dance-lessons-coach/pkg/config"
+
+	_ "github.com/lib/pq"
+)
+
+// TestNewPostgresRepositoryFromDSN_SchemaIsolation verifies that the factory
+// + BuildSchemaIsolatedDSN combo produces a repository whose AutoMigrate
+// creates the users table inside a per-scenario schema (NOT public).
+//
+// This is the foundation block for parallel-safe BDD tests (T12).
+// Wiring it into the BDD testserver's SetupScenarioSchema is a follow-up.
+//
+// Skipped if Postgres is not available (no env vars / connection refused).
+func TestNewPostgresRepositoryFromDSN_SchemaIsolation(t *testing.T) {
+	host := os.Getenv("DLC_DATABASE_HOST")
+	if host == "" {
+		t.Skip("DLC_DATABASE_HOST not set, skipping integration test")
+	}
+
+	cfg := &config.Config{}
+	cfg.Database.Host = host
+	cfg.Database.Port = parsePortOrDefault(os.Getenv("DLC_DATABASE_PORT"), 5432)
+	cfg.Database.User = envOr("DLC_DATABASE_USER", "postgres")
+	cfg.Database.Password = envOr("DLC_DATABASE_PASSWORD", "postgres")
+	cfg.Database.Name = envOr("DLC_DATABASE_NAME", "dance_lessons_coach_bdd_test")
+	cfg.Database.SSLMode = envOr("DLC_DATABASE_SSL_MODE", "disable")
+
+	schemaName := "test_isolated_dsn_factory"
+
+	// Open default repo (public schema) just to manage the schema lifecycle
+	defaultRepo, err := NewPostgresRepository(cfg)
+	if err != nil {
+		t.Skipf("Postgres unavailable: %v", err)
+	}
+	defer defaultRepo.Close()
+
+	// Drop schema if it exists from a previous run
+	if err := defaultRepo.db.Exec(fmt.Sprintf("DROP SCHEMA IF EXISTS %s CASCADE", schemaName)).Error; err != nil {
+		t.Fatalf("DROP SCHEMA setup failed: %v", err)
+	}
+	defer func() {
+		_ = defaultRepo.db.Exec(fmt.Sprintf("DROP SCHEMA IF EXISTS %s CASCADE", schemaName)).Error
+	}()
+
+	// CREATE SCHEMA
+	if err := defaultRepo.db.Exec(fmt.Sprintf("CREATE SCHEMA %s", schemaName)).Error; err != nil {
+		t.Fatalf("CREATE SCHEMA failed: %v", err)
+	}
+
+	// Now use the factory to open a repo whose connection has search_path=schemaName
+	dsn := BuildSchemaIsolatedDSN(cfg, schemaName)
+	isolatedRepo, err := NewPostgresRepositoryFromDSN(cfg, dsn)
+	if err != nil {
+		t.Fatalf("NewPostgresRepositoryFromDSN failed: %v", err)
+	}
+	defer isolatedRepo.Close()
+
+	// Verify the users table now exists in our schema (not just in public)
+	var count int64
+	q := fmt.Sprintf("SELECT count(*) FROM information_schema.tables WHERE table_schema='%s' AND table_name='users'", schemaName)
+	if err := isolatedRepo.db.Raw(q).Scan(&count).Error; err != nil {
+		t.Fatalf("information_schema query failed: %v", err)
+	}
+	if count != 1 {
+		t.Fatalf("expected users table in schema %s after AutoMigrate, count=%d", schemaName, count)
+	}
+
+	// Verify a CreateUser via the isolated repo writes into the new schema, NOT public
+	u := &User{Username: "isolated_factory_user", PasswordHash: "x"}
+	if err := isolatedRepo.CreateUser(context.Background(), u); err != nil {
+		t.Fatalf("CreateUser via isolated repo failed: %v", err)
+	}
+
+	var publicCount int64
+	if err := defaultRepo.db.Raw(fmt.Sprintf("SELECT count(*) FROM public.users WHERE username='%s'", u.Username)).Scan(&publicCount).Error; err != nil {
+		t.Fatalf("query public.users failed: %v", err)
+	}
+	if publicCount != 0 {
+		t.Fatalf("isolation leak: expected 0 rows in public.users for username=%s, got %d", u.Username, publicCount)
+	}
+
+	var schemaCount int64
+	if err := isolatedRepo.db.Raw(fmt.Sprintf("SELECT count(*) FROM %s.users WHERE username='%s'", schemaName, u.Username)).Scan(&schemaCount).Error; err != nil {
+		t.Fatalf("query schema users failed: %v", err)
+	}
+	if schemaCount != 1 {
+		t.Fatalf("expected 1 row in %s.users, got %d", schemaName, schemaCount)
+	}
+}
+
+// envOr returns the env var value or the fallback if empty.
+func envOr(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}
+
+// parsePortOrDefault parses a port string or returns the fallback.
+func parsePortOrDefault(s string, fallback int) int {
+	if s == "" {
+		return fallback
+	}
+	var n int
+	_, err := fmt.Sscanf(s, "%d", &n)
+	if err != nil || n <= 0 {
+		return fallback
+	}
+	return n
+}