fix(model): 为 CelestiaSession 补全默认值以支持 SwiftData 迁移

CelestiaTrace/Models/CelestiaSession.swift

@@ -11,8 +11,11 @@ final class CelestiaSession {
     var isSynced: Bool
     var cloudSessionId: String?
     var ownerUserID: String?
-    var serverRevision: Int64
-    var syncStateRaw: String
+    // Defaults must live on the persisted properties so SwiftData can backfill
+    // existing rows during a lightweight migration. Initializer defaults only
+    // apply to newly-created model instances.
+    var serverRevision: Int64 = 0
+    var syncStateRaw: String = SessionSyncState.localOnly.rawValue
     var lastSyncError: String?
     var lastSyncedAt: Date?