Implement multi-threading for concurrant downloads

[nazwr]

Goal

Enable concurrent downloads using multi-threading to significantly speed up downloading multiple shows or tracks.

Current Behavior

Downloads happen sequentially:

• One show at a time (when multiple shows are provided via Allow multiple shows to be passed at once #10)
• One track at a time within each show
• Large shows or batches can take a long time

Proposed Behavior

Use Ruby threading to download multiple items concurrently:

• Download multiple shows in parallel (e.g., 3 shows at once)
• Download multiple tracks in parallel within a show (e.g., 5 tracks at once)
• Configurable thread pool size (e.g., --threads 5)

Technical Approach

Phase 1: Parallel tracks within a show

# In Show#download_tracks or similar
require 'thread/pool' # or similar gem

pool = Thread.pool(thread_count)
@tracks.each do |track|
  pool.process do
    downloader.get(base_url, track)
  end
end
pool.shutdown

Phase 2: Parallel shows

# In DeadList#run when handling multiple shows
show_ids.each do |show_id|
  pool.process do
    # Create show, download tracks
  end
end

Considerations

Thread Safety:

• Logger is thread-safe by default ✓
• File I/O needs to ensure unique filenames (already handled) ✓
• Network connections (HTTParty) are thread-safe ✓

Error Handling:

• Errors in one thread shouldn't crash others
• Collect and report errors after all threads complete
• Maintain existing error messages

Resource Limits:

• Don't overwhelm archive.org servers (be a good citizen)
• Reasonable default thread count (3-5)
• Allow users to configure via --threads N flag

Progress Tracking:

• Works great with Replace logging messages about shows to progress bar - ala pacman downloads #46 (progress bars) - show multiple progress bars
• Logger output needs to handle concurrent messages (already does)

Dependencies

• Builds on Allow multiple shows to be passed at once #10 (multiple shows support) - threading makes even more sense with batch downloads
• Enhances Replace logging messages about shows to progress bar - ala pacman downloads #46 (progress bars) - visualizing concurrent downloads is compelling

Implementation Phases

1. Basic threading for tracks - Parallel track downloads within a show
2. Add --threads flag - Let users control concurrency
3. Parallel shows - Download multiple shows concurrently
4. Polish - Error aggregation, proper shutdown handling

Benefits

• 3-5x speedup for shows with many tracks
• Much faster batch downloads of multiple shows
• Better utilization of available bandwidth
• Still respectful to archive.org (configurable limits)

Related Issues

• Allow multiple shows to be passed at once #10 - Multiple shows support (prerequisite)
• Replace logging messages about shows to progress bar - ala pacman downloads #46 - Progress bars (great complement)