feat(metadata): SQLite backend, storage reconciliation and local defaults

- pluggable per-domain Repository interfaces; PostgreSQL and SQLite impls
- SQLite on modernc.org/sqlite (WAL, busy_timeout, foreign_keys, immediate tx)
- separate migrations/sqlite and db.MigrateSQLite; store.NewPostgres/NewSQLite
- cmd/reconcile compares object storage with the journal (report, then --delete)
- storage.Backend.List and photos.Repository.AllStorageKeys
- DB_BACKEND/SQLITE_PATH and STORAGE_BACKEND/FS_ROOT selectable; local defaults
  (sqlite ./galo.db, fs ./data)
- Makefile reconcile/reconcile-delete; docs and .env.example updated

backend/internal/db/migrate.go

@@ -8,8 +8,10 @@ import (
 	"database/sql"
 	"github.com/golang-migrate/migrate/v4"
 	"github.com/golang-migrate/migrate/v4/database/pgx/v5"
+	sqlitemig "github.com/golang-migrate/migrate/v4/database/sqlite"
 	"github.com/golang-migrate/migrate/v4/source/iofs"
 	_ "github.com/jackc/pgx/v5/stdlib"
+	_ "modernc.org/sqlite"
 )
 
 // Migrate applies all up migrations from the given fs (root contains *.sql files).
@@ -39,3 +41,32 @@ func Migrate(dsn string, migrations fs.FS) error {
 	}
 	return nil
 }
+
+// MigrateSQLite applies all up migrations from the given fs to a SQLite database
+// at path (creating it if needed).
+func MigrateSQLite(path string, migrations fs.FS) error {
+	source, err := iofs.New(migrations, ".")
+	if err != nil {
+		return fmt.Errorf("migrations source: %w", err)
+	}
+
+	sqlDB, err := sql.Open("sqlite", path)
+	if err != nil {
+		return fmt.Errorf("open sql db: %w", err)
+	}
+	defer sqlDB.Close()
+
+	driver, err := sqlitemig.WithInstance(sqlDB, &sqlitemig.Config{})
+	if err != nil {
+		return fmt.Errorf("migrate driver: %w", err)
+	}
+
+	m, err := migrate.NewWithInstance("iofs", source, "sqlite", driver)
+	if err != nil {
+		return fmt.Errorf("migrate instance: %w", err)
+	}
+	if err := m.Up(); err != nil && !errors.Is(err, migrate.ErrNoChange) {
+		return fmt.Errorf("migrate up: %w", err)
+	}
+	return nil
+}

backend/internal/db/migrate_sqlite_test.go

@@ -0,0 +1,37 @@
+package db
+
+import (
+	"io/fs"
+	"path/filepath"
+	"testing"
+
+	assets "shared-album"
+)
+
+func TestMigrateSQLite(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "meta.db")
+	migFS, err := fs.Sub(assets.Migrations, "migrations/sqlite")
+	if err != nil {
+		t.Fatalf("migrations fs: %v", err)
+	}
+	if err := MigrateSQLite(path, migFS); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	if err := MigrateSQLite(path, migFS); err != nil {
+		t.Fatalf("migrate again: %v", err)
+	}
+
+	db, err := OpenSQLite(path)
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	defer db.Close()
+
+	var n int
+	if err := db.QueryRow(`SELECT count(*) FROM sqlite_master WHERE type='table' AND name='objects'`).Scan(&n); err != nil {
+		t.Fatalf("query: %v", err)
+	}
+	if n != 1 {
+		t.Fatalf("objects table missing after migration")
+	}
+}

backend/internal/db/sqlite.go

@@ -0,0 +1,31 @@
+package db
+
+import (
+	"database/sql"
+	"fmt"
+	"strings"
+
+	_ "modernc.org/sqlite"
+)
+
+// OpenSQLite opens (creating if needed) a SQLite database tuned for a single
+// writer with concurrent readers: WAL journalling, a busy timeout so writers
+// wait instead of failing, and enforced foreign keys.
+func OpenSQLite(path string) (*sql.DB, error) {
+	if strings.TrimSpace(path) == "" {
+		return nil, fmt.Errorf("sqlite path is required")
+	}
+	dsn := fmt.Sprintf(
+		"file:%s?_pragma=journal_mode(WAL)&_pragma=busy_timeout(5000)&_pragma=foreign_keys(1)&_pragma=synchronous(NORMAL)&_txlock=immediate",
+		path,
+	)
+	db, err := sql.Open("sqlite", dsn)
+	if err != nil {
+		return nil, fmt.Errorf("open sqlite: %w", err)
+	}
+	if err := db.Ping(); err != nil {
+		_ = db.Close()
+		return nil, fmt.Errorf("ping sqlite: %w", err)
+	}
+	return db, nil
+}

backend/internal/db/sqlite_test.go

@@ -0,0 +1,45 @@
+package db
+
+import (
+	"context"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestOpenSQLite(t *testing.T) {
+	ctx := context.Background()
+	db, err := OpenSQLite(filepath.Join(t.TempDir(), "test.db"))
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	defer db.Close()
+
+	if err := db.PingContext(ctx); err != nil {
+		t.Fatalf("ping: %v", err)
+	}
+
+	var mode string
+	if err := db.QueryRowContext(ctx, "PRAGMA journal_mode").Scan(&mode); err != nil {
+		t.Fatalf("journal_mode: %v", err)
+	}
+	if !strings.EqualFold(mode, "wal") {
+		t.Fatalf("journal_mode = %q, want wal", mode)
+	}
+
+	var fk int
+	if err := db.QueryRowContext(ctx, "PRAGMA foreign_keys").Scan(&fk); err != nil {
+		t.Fatalf("foreign_keys: %v", err)
+	}
+	if fk != 1 {
+		t.Fatalf("foreign_keys = %d, want 1", fk)
+	}
+
+	var busy int
+	if err := db.QueryRowContext(ctx, "PRAGMA busy_timeout").Scan(&busy); err != nil {
+		t.Fatalf("busy_timeout: %v", err)
+	}
+	if busy != 5000 {
+		t.Fatalf("busy_timeout = %d, want 5000", busy)
+	}
+}