Integrate IndieBackup for snapshot backup and restore

Adds a local ZIP backup/restore of the iCloud document tree via the
IndieBackup package, surfaced in Settings with retention controls. A
restore suspends the sync observer, mirrors the files, then rebuilds the
SwiftData cache; opening a shared .workoutsbackup file restores it. The
engine exposes the container Documents root and a restore lifecycle
(isRestoring guards a concurrent connect), and the backup file type is
registered for open-in-place.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Workouts/WorkoutsApp.swift

@@ -41,5 +41,12 @@ struct WorkoutsApp: App {
             .environment(services.liveRunState)
             .modelContainer(services.container)
             .task { await services.bootstrap() }
+            // Tap a shared ".workoutsbackup" file (Files, AirDrop, share sheet) to
+            // restore it. The controller stops the sync observer for the restore and
+            // rebuilds the cache afterward (see AppBackupConfiguration).
+            .onOpenURL { url in
+                guard url.pathExtension == "workoutsbackup" else { return }
+                Task { try? await services.backupController.restoreBackup(from: url) }
+            }
     }
 }