perf: move heavy work off main actor, throttle progress updates, add signposts

BackupEngine:
- fetchAssets() runs in Task.detached — prevents 100-500ms UI freeze during
  PHFetchResult enumeration on large libraries
- Manifest JSON decode + ManifestIndex Set construction run in Task.detached
- pendingAssets filter runs in Task.detached — O(N) array filter off main thread
- Progress updates throttled to 8 Hz via ProgressThrottle — eliminates the
  continuous SwiftUI full-tree redraws caused by per-byte upload callbacks
- bytesSoFar captured by value in upload closure — avoids mutable var capture
- reserveCapacity on manifestEntries array
- os_signpost intervals on FetchAssets, NASConnect, ManifestLoad,
  BuildPendingQueue, UploadLoop, UploadFile for Instruments profiling

BackupStatusService:
- countSafe() marked nonisolated — runs in Task.detached off main actor,
  keeping touch/animation responsive during PHFetchResult batch enumeration
- ManifestIndex JSON decode + Set construction run in Task.detached
- Bootstrap ManifestIndex(nasListing:) construction off main actor
- writeManifest JSON decode/encode/merge off main actor
- os_signpost intervals on Reconcile, ManifestLoad, CountSafe

Model types:
- ManifestIndex, ManifestEntry, PhotoAsset, BackupFilter marked Sendable —
  safe to pass across task boundaries without warnings

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

Core/Protocols/PhotoLibraryProtocol.swift

@@ -1,7 +1,7 @@
 import Foundation
 import Photos
 
-struct PhotoAsset {
+struct PhotoAsset: Sendable {
     let localIdentifier: String
     let filename: String
     let creationDate: Date?
@@ -20,7 +20,7 @@ protocol PhotoLibraryProtocol: AnyObject {
     func exportAsset(_ asset: PhotoAsset) async throws -> URL
 }
 
-struct BackupFilter: Codable {
+struct BackupFilter: Codable, Sendable {
     var includePhotos: Bool = true
     var includeVideos: Bool = true
     var includeScreenshots: Bool = false