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/Resources/Info-iOS.plist

@@ -57,5 +57,42 @@
 			<string>Any</string>
 		</dict>
 	</dict>
+	<key>LSSupportsOpeningDocumentsInPlace</key>
+	<true/>
+	<key>CFBundleDocumentTypes</key>
+	<array>
+		<dict>
+			<key>CFBundleTypeName</key>
+			<string>Workouts Backup</string>
+			<key>CFBundleTypeRole</key>
+			<string>Editor</string>
+			<key>LSHandlerRank</key>
+			<string>Owner</string>
+			<key>LSItemContentTypes</key>
+			<array>
+				<string>dev.rzen.indie.workoutsbackup</string>
+			</array>
+		</dict>
+	</array>
+	<key>UTExportedTypeDeclarations</key>
+	<array>
+		<dict>
+			<key>UTTypeIdentifier</key>
+			<string>dev.rzen.indie.workoutsbackup</string>
+			<key>UTTypeDescription</key>
+			<string>Workouts Backup</string>
+			<key>UTTypeConformsTo</key>
+			<array>
+				<string>public.data</string>
+			</array>
+			<key>UTTypeTagSpecification</key>
+			<dict>
+				<key>public.filename-extension</key>
+				<array>
+					<string>workoutsbackup</string>
+				</array>
+			</dict>
+		</dict>
+	</array>
 </dict>
 </plist>