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/Views/Settings/SettingsView.swift

@@ -8,6 +8,7 @@
 import SwiftUI
 import SwiftData
 import IndieAbout
+import IndieBackup
 
 struct SettingsView: View {
     @Environment(SyncEngine.self) private var sync
@@ -132,6 +133,9 @@ struct SettingsView: View {
                     Text("iCloud Sync")
                 }
 
+                // MARK: - Backups Section
+                BackupsSectionView(controller: services.backupController)
+
                 // MARK: - Developer Section (debug builds only)
                 #if DEBUG
                 Section {