Implement storage architecture from ai_blueprint.md

Primary sync: replace PersistenceActor JSON file with SwiftData + CloudKit
- Add SavedCardModel (@Model class) and PersistenceController (ModelContainer
  with .automatic CloudKit, fallback to local). BackgroundPersistenceActor
  (@ModelActor) handles all DB I/O off the main thread.
- One-time migration imports user_collection.json into SwiftData and renames
  the original file to prevent re-import.
- Inject modelContainer into SwiftUI environment in IYmtgApp.

Image storage: Documents/UserContent/ subfolder (blueprint requirement)
- ImageManager.dir now targets iCloud Documents/UserContent/ (or local equiv).
- migrateImagesToUserContent() moves existing JPGs to the new subfolder on
  first launch; called during the SwiftData migration.

Firebase: demoted to optional manual backup (metadata only, no images)
- Remove all automatic CloudEngine.save/delete/batchUpdatePrices calls from
  CollectionViewModel mutations.
- Add backupAllToFirebase() for user-triggered metadata sync.
- Add isFirebaseBackupEnabled to AppConfig (default false).
- Add Cloud Backup section in Library settings with iCloud vs Firebase
  explanation and "Backup Metadata to Firebase Now" button.

Also: full modular refactor (Data/, Features/, Services/ directories) and
README updated with CloudKit setup steps and revised release checklist.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

IYmtg_App_iOS/Services/Utilities/StoreEngine.swift

@@ -0,0 +1,55 @@
+import StoreKit
+
+@MainActor
+class StoreEngine: ObservableObject {
+    @Published var products: [Product] = []
+    @Published var showThankYou = false
+    var transactionListener: Task<Void, Never>? = nil
+
+    init() {
+        // FIX: Added [weak self] to prevent retain cycle
+        transactionListener = Task.detached { [weak self] in
+            for await result in Transaction.updates {
+                do {
+                    guard let self = self else { return }
+                    let transaction = try self.checkVerified(result)
+                    await transaction.finish()
+                    await MainActor.run { self.showThankYou = true }
+                } catch {}
+            }
+        }
+    }
+
+    deinit { transactionListener?.cancel() }
+
+    func loadProducts() async {
+        do {
+            let p = try await Product.products(for: AppConfig.tipJarProductIDs)
+            self.products = p
+            #if DEBUG
+            if p.isEmpty { print("⚠️ StoreEngine: No products found. Verify IAP ID in AppConfig.") }
+            #endif
+        } catch { print("Store Error: \(error)") }
+    }
+
+    func purchase(_ product: Product) async {
+        guard let result = try? await product.purchase() else { return }
+        switch result {
+        case .success(let verification):
+            if let transaction = try? checkVerified(verification) {
+                await transaction.finish()
+                self.showThankYou = true
+            }
+        case .pending, .userCancelled: break
+        @unknown default: break
+        }
+    }
+
+    nonisolated func checkVerified<T>(_ result: VerificationResult<T>) throws -> T {
+        switch result {
+        case .unverified: throw StoreError.failedVerification
+        case .verified(let safe): return safe
+        }
+    }
+    enum StoreError: Error { case failedVerification }
+}