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/sqliteutil/sqliteutil.go

@@ -0,0 +1,50 @@
+// Package sqliteutil holds small helpers shared by the SQLite repositories:
+// timestamp conversion and dynamic IN-lists.
+package sqliteutil
+
+import (
+	"database/sql"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// TimeLayout matches the SQLite column defaults produced by strftime with %f.
+const TimeLayout = "2006-01-02T15:04:05.000Z"
+
+// FormatTime renders a time in the canonical TEXT format used by the schema.
+func FormatTime(t time.Time) string { return t.UTC().Format(TimeLayout) }
+
+// ParseTime parses a canonical timestamp, tolerating the RFC3339 variants the
+// driver may return. An empty or unparseable value yields the zero time.
+func ParseTime(s string) time.Time {
+	if s == "" {
+		return time.Time{}
+	}
+	if t, err := time.Parse(TimeLayout, s); err == nil {
+		return t
+	}
+	if t, err := time.Parse(time.RFC3339Nano, s); err == nil {
+		return t
+	}
+	return time.Time{}
+}
+
+// ParseNullTime converts a nullable TEXT timestamp into *time.Time.
+func ParseNullTime(ns sql.NullString) *time.Time {
+	if !ns.Valid || ns.String == "" {
+		return nil
+	}
+	t := ParseTime(ns.String)
+	return &t
+}
+
+// Placeholders returns an IN-list placeholder group of n items starting at
+// index start, e.g. Placeholders(3, 2) -> "$3,$4".
+func Placeholders(start, n int) string {
+	parts := make([]string, n)
+	for i := 0; i < n; i++ {
+		parts[i] = "$" + strconv.Itoa(start+i)
+	}
+	return strings.Join(parts, ",")
+}