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>

Services/PhotoLibraryService.swift

@@ -68,6 +68,39 @@ final class PhotoLibraryService: PhotoLibraryProtocol {
         return assets
     }
 
+    /// Returns a raw PHFetchResult for memory-safe reconciliation.
+    /// No PhotoAsset array is created — caller enumerates in batches.
+    func fetchResultForReconciliation(filter: BackupFilter) -> PHFetchResult<PHAsset> {
+        let options = PHFetchOptions()
+        options.sortDescriptors = [NSSortDescriptor(key: "creationDate", ascending: true)]
+
+        var subpredicates: [NSPredicate] = []
+        var mediaTypes: [PHAssetMediaType] = []
+        if filter.includePhotos { mediaTypes.append(.image) }
+        if filter.includeVideos { mediaTypes.append(.video) }
+
+        if !mediaTypes.isEmpty {
+            subpredicates.append(NSPredicate(
+                format: "mediaType IN %@",
+                mediaTypes.map(\.rawValue)
+            ))
+        }
+        if !filter.includeScreenshots {
+            subpredicates.append(NSPredicate(
+                format: "NOT ((mediaSubtype & %d) != 0)",
+                PHAssetMediaSubtype.photoScreenshot.rawValue
+            ))
+        }
+
+        if subpredicates.count > 1 {
+            options.predicate = NSCompoundPredicate(andPredicateWithSubpredicates: subpredicates)
+        } else {
+            options.predicate = subpredicates.first
+        }
+
+        return PHAsset.fetchAssets(with: options)
+    }
+
     func exportAsset(_ asset: PhotoAsset) async throws -> URL {
         let fetchResult = PHAsset.fetchAssets(
             withLocalIdentifiers: [asset.localIdentifier],