Make backup status reconciliation memory-safe

Replace full [PhotoAsset] array enumeration with PHFetchResult batch
enumeration (autoreleasepool + Task.yield per batch) to keep memory
bounded during reconciliation. Introduce ManifestIndex with two O(1)
Set<String> lookups (localIdentifier + lowercased filename) instead of
O(n×m) linear scans. Scope manifest Data+struct inside buildManifestIndex
so they are released by ARC before the comparison loop begins. Add
UIApplication.didReceiveMemoryWarningNotification handler to cancel
in-flight tasks under memory pressure. Separate debounceTask from
refreshTask to avoid cancelling long-running reconciles on every photo
library event. fetchResultForReconciliation returns PHFetchResult
directly — no [PhotoAsset] array is ever allocated during reconciliation.

Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>

Core/Models/BackupManifest.swift

@@ -1,4 +1,54 @@
 import Foundation
+import Photos
+
+// MARK: — ManifestIndex
+// Lightweight comparison structure — two Sets for O(1) lookups.
+// Built from BackupManifest and then the manifest is released.
+struct ManifestIndex {
+    let localIdentifiers: Set<String>    // PHAsset.localIdentifier (primary key)
+    let lowercasedFilenames: Set<String> // filename fallback for legacy/bootstrap entries
+    let totalCount: Int                  // non-hidden file count for NAS Archive stat
+
+    var isFilenameOnly: Bool { localIdentifiers.isEmpty }
+
+    func matches(localIdentifier id: String) -> Bool {
+        !id.isEmpty && localIdentifiers.contains(id)
+    }
+
+    func matches(filename: String) -> Bool {
+        lowercasedFilenames.contains(filename.lowercased())
+    }
+
+    // Build from decoded manifest — manifest released by caller after this returns
+    init(manifest: BackupManifest) {
+        var ids   = Set<String>(minimumCapacity: manifest.entries.count)
+        var names = Set<String>(minimumCapacity: manifest.entries.count)
+        var count = 0
+        for entry in manifest.entries {
+            if !entry.filename.hasPrefix(".") { count += 1 }
+            if !entry.localIdentifier.isEmpty { ids.insert(entry.localIdentifier) }
+            names.insert(entry.filename.lowercased())
+        }
+        self.localIdentifiers   = ids
+        self.lowercasedFilenames = names
+        self.totalCount = count
+    }
+
+    // Build from NAS directory listing (bootstrap — no localIdentifiers known)
+    init(nasListing items: [NASItem]) {
+        var names = Set<String>()
+        var count = 0
+        for item in items where !item.isDirectory && item.name != BackupManifest.remoteFilename {
+            if !item.name.hasPrefix(".") { count += 1 }
+            names.insert(item.name.lowercased())
+        }
+        self.localIdentifiers   = []
+        self.lowercasedFilenames = names
+        self.totalCount = count
+    }
+}
+
+// MARK: — ManifestEntry
 
 struct ManifestEntry: Codable {
     let localIdentifier: String   // PHAsset.localIdentifier — primary key