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/reconcile/reconcile_test.go

@@ -0,0 +1,41 @@
+package reconcile
+
+import (
+	"reflect"
+	"testing"
+	"time"
+
+	"shared-album/internal/storage"
+)
+
+func TestClassify(t *testing.T) {
+	now := time.Now()
+	objects := []storage.ObjectInfo{
+		{Key: "objects/a/original.jpg", ModTime: now},
+		{Key: "objects/b/original.jpg", ModTime: now},
+	}
+	journaled := []string{"objects/b/original.jpg", "objects/c/original.jpg"}
+
+	got := Classify(objects, journaled)
+	if !reflect.DeepEqual(got.Orphans, []string{"objects/a/original.jpg"}) {
+		t.Fatalf("orphans = %v", got.Orphans)
+	}
+	if !reflect.DeepEqual(got.Missing, []string{"objects/c/original.jpg"}) {
+		t.Fatalf("missing = %v", got.Missing)
+	}
+}
+
+func TestDeletableRespectsAgeAndPrefix(t *testing.T) {
+	now := time.Now()
+	objects := []storage.ObjectInfo{
+		{Key: "objects/old/original.jpg", ModTime: now.Add(-2 * time.Hour)},
+		{Key: "objects/new/original.jpg", ModTime: now.Add(-1 * time.Minute)},
+		{Key: "misc/other.bin", ModTime: now.Add(-2 * time.Hour)},
+	}
+	res := Result{Orphans: []string{"objects/old/original.jpg", "objects/new/original.jpg", "misc/other.bin"}}
+
+	got := Deletable(objects, res, "objects/", time.Hour, now)
+	if !reflect.DeepEqual(got, []string{"objects/old/original.jpg"}) {
+		t.Fatalf("deletable = %v, want only the old in-prefix orphan", got)
+	}
+}