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/Cloud/CloudEngine.swift

@@ -0,0 +1,70 @@
+import FirebaseFirestore
+import FirebaseAuth
+
+// MARK: - CLOUD ENGINE
+class CloudEngine {
+    static var db: Firestore? {
+        return FirebaseApp.app() != nil ? Firestore.firestore() : nil
+    }
+
+    static func signInSilently() {
+        guard FirebaseApp.app() != nil else { return }
+        if Auth.auth().currentUser == nil { Auth.auth().signInAnonymously() }
+    }
+
+    static func save(card: SavedCard) async {
+        guard let db = db, let uid = Auth.auth().currentUser?.uid else { return }
+        let data: [String: Any] = [
+            "name": card.name,
+            "set": card.setCode,
+            "num": card.collectorNumber,
+            "val": card.currentValuation ?? 0.0,
+            "cond": card.condition,
+            "foil": card.foilType,
+            "coll": card.collectionName,
+            "loc": card.storageLocation,
+            "grade": card.grade ?? "",
+            "svc": card.gradingService ?? "",
+            "cert": card.certNumber ?? "",
+            "date": card.dateAdded,
+            "curr": card.currencyCode ?? "USD",
+            "rarity": card.rarity ?? "",
+            "colors": card.colorIdentity ?? [],
+            "ser": card.isSerialized ?? false,
+            "img": card.imageFileName,
+            "custom": card.isCustomValuation
+        ]
+        do {
+            try await db.collection("users").document(uid).collection("inventory").document(card.id.uuidString).setData(data, merge: true)
+        } catch { print("Cloud Save Error: \(error)") }
+    }
+
+    static func delete(card: SavedCard) async {
+        guard let db = db, let uid = Auth.auth().currentUser?.uid else { return }
+        do {
+            try await db.collection("users").document(uid).collection("inventory").document(card.id.uuidString).delete()
+        } catch { print("Cloud Delete Error: \(error)") }
+    }
+
+    static func batchUpdatePrices(cards: [SavedCard]) async {
+        guard let db = db, let uid = Auth.auth().currentUser?.uid else { return }
+        let ref = db.collection("users").document(uid).collection("inventory")
+
+        let chunks = stride(from: 0, to: cards.count, by: 400).map { Array(cards[$0..<min($0 + 400, cards.count)]) }
+        for chunk in chunks {
+            let batch = db.batch()
+            for card in chunk {
+                let data: [String: Any] = [
+                    "val": card.currentValuation ?? 0.0,
+                    "curr": card.currencyCode ?? "USD",
+                    "rarity": card.rarity ?? "",
+                    "colors": card.colorIdentity ?? []
+                ]
+                batch.setData(data, forDocument: ref.document(card.id.uuidString), merge: true)
+            }
+            do {
+                try await batch.commit()
+            } catch { print("Batch Update Error: \(error)") }
+        }
+    }
+}