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.go

@@ -0,0 +1,74 @@
+// Package reconcile computes the difference between the object storage and the
+// metadata journal so orphaned blobs can be reported and, on request, removed.
+package reconcile
+
+import (
+	"sort"
+	"strings"
+	"time"
+
+	"shared-album/internal/storage"
+)
+
+// Result holds the two kinds of divergence between storage and the journal.
+type Result struct {
+	// Orphans are keys present in storage but absent from the journal; they are
+	// candidates for deletion.
+	Orphans []string
+	// Missing are keys present in the journal but absent from storage; they are
+	// reported only and never modified.
+	Missing []string
+}
+
+// Classify compares the listed storage objects with the journaled keys.
+func Classify(objects []storage.ObjectInfo, journaled []string) Result {
+	have := make(map[string]struct{}, len(journaled))
+	for _, k := range journaled {
+		have[k] = struct{}{}
+	}
+	seen := make(map[string]struct{}, len(objects))
+	var orphans []string
+	for _, o := range objects {
+		if _, dup := seen[o.Key]; dup {
+			continue
+		}
+		seen[o.Key] = struct{}{}
+		if _, ok := have[o.Key]; !ok {
+			orphans = append(orphans, o.Key)
+		}
+	}
+	var missing []string
+	for _, k := range journaled {
+		if _, ok := seen[k]; !ok {
+			missing = append(missing, k)
+		}
+	}
+	sort.Strings(orphans)
+	sort.Strings(missing)
+	return Result{Orphans: orphans, Missing: missing}
+}
+
+// Deletable selects orphaned keys that are safe to remove: within prefix and
+// older than minAge (a guard against racing an in-flight upload whose row is
+// not written yet).
+func Deletable(objects []storage.ObjectInfo, res Result, prefix string, minAge time.Duration, now time.Time) []string {
+	orphans := make(map[string]struct{}, len(res.Orphans))
+	for _, k := range res.Orphans {
+		orphans[k] = struct{}{}
+	}
+	var out []string
+	for _, o := range objects {
+		if _, ok := orphans[o.Key]; !ok {
+			continue
+		}
+		if prefix != "" && !strings.HasPrefix(o.Key, prefix) {
+			continue
+		}
+		if now.Sub(o.ModTime) < minAge {
+			continue
+		}
+		out = append(out, o.Key)
+	}
+	sort.Strings(out)
+	return out
+}