feat(migrate): configure embedded Goose providers

Use provider-local SQL and Go migrations, injected slog logging, and PostgreSQL session locking. Exercise timestamp migration flows and failures with SQLite and PostgreSQL, including concurrent startup, and require PostgreSQL integration coverage in CI.

Refs #7

migrate/migrate.go

@@ -0,0 +1,56 @@
+// Package migrate configures Goose providers for embedded application migrations.
+// Applications own their database, migration files, and startup ordering.
+package migrate
+
+import (
+	"database/sql"
+	"embed"
+	"fmt"
+	"io/fs"
+	"log/slog"
+
+	"github.com/pressly/goose/v3"
+	"github.com/pressly/goose/v3/lock"
+)
+
+// New returns a provider for SQL files in dir and optional provider-local Go
+// migrations. Pass "." for files at the embedded root and a nonnil logger.
+// Construction does not connect to db; the caller owns and closes db.
+//
+// PostgreSQL uses Goose's session advisory locker with its default lock ID and
+// retry limits. Other dialects require the caller to serialize migration runs
+// across providers/processes. PostgreSQL connections must preserve session
+// affinity (transaction-pooling proxies cannot safely hold session locks).
+//
+// Use the returned Provider's Up, UpTo, DownTo, Status, GetDBVersion, GetVersions,
+// and HasPending methods directly. Versions may be sequential or timestamps;
+// Goose orders and tracks them. Global Go migration registration is ignored.
+func New(db *sql.DB, dialect goose.Dialect, files embed.FS, dir string, logger *slog.Logger, migrations ...*goose.Migration) (*goose.Provider, error) {
+	for i, migration := range migrations {
+		if migration == nil {
+			return nil, fmt.Errorf("migrate: Go migration %d must not be nil", i)
+		}
+	}
+	root, err := fs.Sub(files, dir)
+	if err != nil {
+		return nil, fmt.Errorf("migrate: directory %q: %w", dir, err)
+	}
+	opts := []goose.ProviderOption{
+		goose.WithDisableGlobalRegistry(true),
+		goose.WithGoMigrations(migrations...),
+		goose.WithSlog(logger),
+		goose.WithVerbose(true),
+	}
+	if dialect == goose.DialectPostgres {
+		locker, err := lock.NewPostgresSessionLocker()
+		if err != nil {
+			return nil, fmt.Errorf("migrate: postgres locker: %w", err)
+		}
+		opts = append(opts, goose.WithSessionLocker(locker))
+	}
+	provider, err := goose.NewProvider(dialect, db, root, opts...)
+	if err != nil {
+		return nil, fmt.Errorf("migrate: new provider: %w", err)
+	}
+	return provider, nil
+}