feat: verify embedded backup media checksums

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/application/ports.dart

@@ -200,6 +200,7 @@ final class EmbeddedBackupMediaFile {
     required this.fileName,
     required this.mimeType,
     required this.sizeBytes,
+    required this.sha256,
     required this.base64,
   });
 
@@ -208,6 +209,7 @@ final class EmbeddedBackupMediaFile {
   final String fileName;
   final String? mimeType;
   final int sizeBytes;
+  final String sha256;
   final String base64;
 }