Sync starred models across machines; add automatic backup scheduling

Favorites now push/pull through a small oai_favorites.json file in the
same iCloud Drive folder used by Settings > Backup, reconciled by
last-write-wins timestamp on launch and app-become-active. Also adds
an Off/Daily/Weekly frequency picker so the full settings backup can
run itself (checked at launch and hourly) instead of requiring a
manual "Back Up Now" click every time.

oAI/Services/SettingsService.swift

@@ -520,10 +520,33 @@ class SettingsService {
         }
     }
 
+    /// ISO8601 timestamp of the last local change to favoriteModelIds — used to
+    /// resolve last-write-wins conflicts when syncing favorites across machines.
+    var favoriteModelsUpdatedAt: String {
+        get { cache["favoriteModelsUpdatedAt"] ?? "" }
+        set {
+            cache["favoriteModelsUpdatedAt"] = newValue
+            DatabaseService.shared.setSetting(key: "favoriteModelsUpdatedAt", value: newValue)
+        }
+    }
+
     func toggleFavoriteModel(_ id: String) {
         var favs = favoriteModelIds
         if favs.contains(id) { favs.remove(id) } else { favs.insert(id) }
         favoriteModelIds = favs
+        favoriteModelsUpdatedAt = ISO8601DateFormatter().string(from: Date())
+        Task { await BackupService.shared.pushFavorites() }
+    }
+
+    // MARK: - Automatic Backup
+
+    /// "manual" (default), "daily", or "weekly"
+    var autoBackupFrequency: String {
+        get { cache["autoBackupFrequency"] ?? "manual" }
+        set {
+            cache["autoBackupFrequency"] = newValue
+            DatabaseService.shared.setSetting(key: "autoBackupFrequency", value: newValue)
+        }
     }
 
     // MARK: - Anytype MCP Settings