Enhancement: Support batch asset upload (multiple files)

[leogdion]

Overview

Extend uploadAssets() to support uploading multiple files in a single operation for better efficiency.

Current API

// Upload one file at a time
let data1 = try Data(contentsOf: imageURL1)
let result1 = try await service.uploadAssets(data: data1)

let data2 = try Data(contentsOf: imageURL2)
let result2 = try await service.uploadAssets(data: data2)

Proposed API

// Batch upload
let files = [imageURL1, imageURL2, imageURL3]
let results = try await service.uploadAssets(files: files)
// Returns [AssetUploadResult] with tokens for each file

// Or with Data array
let dataArray = [data1, data2, data3]
let results = try await service.uploadAssets(dataArray: dataArray)

Benefits

• Reduced latency: Single request overhead for multiple files
• Better throughput: Network efficiency gains
• Simplified code: Upload all assets in one call
• Atomic operations: All uploads succeed or fail together (optional)

Implementation Notes

• CloudKit API supports multiple files in single multipart request
• Add uploadAssets(files: [URL]) overload
• Add uploadAssets(dataArray: [Data]) overload
• Validate total size across all files (250 MB per file limit)
• Consider parallel upload option for large batches
• Return array of AssetUploadResult maintaining order

Considerations

• Should batch upload be atomic (all or nothing) or best-effort?
• Maximum files per batch (CloudKit limit)?
• Progress reporting for multi-file uploads?

Related

• Implemented in Uploading Assets (assets/upload) #30 (uploadAssets)
• Consider combining with progress tracking enhancement