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

project.yml

@@ -22,6 +22,9 @@ packages:
   IndieSync:
     url: https://git.rzen.dev/rzen/indie-sync.git
     from: "0.3.0"
+  IndieBackup:
+    url: https://git.rzen.dev/rzen/indie-backup.git
+    from: "2.0.0"
 
 # Shared post-build phase — stamps CFBundleVersion (git commit count), BuildDate, and
 # BuildHash into each target's (and dSYM's) Info.plist. Aliased into every code-bearing
@@ -62,6 +65,7 @@ targets:
     dependencies:
       - package: IndieAbout
       - package: IndieSync
+      - package: IndieBackup
       - target: Workouts Watch App
       - target: Workouts Widget
         embed: true