feat: skip local hashing for iCloud assets, use server-computed checksum

Instead of downloading iCloud assets twice (once to hash, once to upload),
skip local hashing for iCloud-only assets and let the server return its
computed SHA1 checksum in the upload response. The mobile app stores this
checksum locally to prevent re-uploads.

Changes:
- Server returns checksum in upload response (created + duplicate)
- iOS native layer tags iCloud-only assets with ICLOUD_ONLY error
- Hash service skips iCloud assets (allowNetworkAccess: false)
- Upload result carries server checksum back to mobile
- Foreground/background upload services store server checksum
- Backup candidates include unhashed assets (onlyHashed: false)

https://claude.ai/code/session_01LEs74WpkJ1gWcJrFBsp1i2

mobile/ios/Runner/Sync/MessagesImpl.swift

@@ -357,6 +357,12 @@ class NativeSyncApiImpl: ImmichPlugin, NativeSyncApi, FlutterPlugin {
           completionHandler: { error in
             let result: HashResult? = switch (error) {
             case let e as PHPhotosError where e.code == .userCancelled: nil
+            case let e as PHPhotosError where e.code == .networkAccessRequired:
+              HashResult(
+                assetId: asset.localIdentifier,
+                error: "ICLOUD_ONLY",
+                hash: nil
+              )
             case let .some(e): HashResult(
               assetId: asset.localIdentifier,
               error: "Failed to hash asset: \(e.localizedDescription)",