feature: add HTTP Range request support for video/audio seeking #68

Summary

JSS currently doesn't support HTTP Range requests, which means video/audio seeking (skipping forward/back) doesn't work.

Current Behavior

When a client requests a byte range (e.g., seeking in a video), JSS ignores the Range header and returns the full file with status 200.

Expected Behavior

JSS should:

1. Check for Range header in GET requests
2. Parse byte range (e.g., bytes=1000-2000 or bytes=1000-)
3. Return partial content with status 206 Partial Content
4. Include proper headers:
   • Content-Range: bytes start-end/total
   • Accept-Ranges: bytes
   • Content-Length: chunksize

Use Cases

• Video seeking - Skip to any point in a video without downloading the whole file
• Audio seeking - Same for audio files
• Resumable downloads - Resume interrupted large file downloads
• Efficient streaming - Only transfer what's needed

Reference Implementation

NSS has this in lib/ldp.mjs:470-490 and lib/handlers/get.mjs:54,124-128:

// Parse range header
if (options.range) {
  const parts = options.range.replace(/bytes=/, '').split('-')
  const start = parseInt(parts[0], 10)
  const end = parts[1] ? parseInt(parts[1], 10) : total - 1
  const chunksize = (end - start) + 1
  contentRange = 'bytes ' + start + '-' + end + '/' + total
  
  // Create read stream with range
  stream = fs.createReadStream(path, { start, end })
}

// Set response headers
if (contentRange) {
  res.status(206)
  res.set({
    'Content-Range': contentRange,
    'Accept-Ranges': 'bytes',
    'Content-Length': chunksize
  })
}

Implementation Notes

For JSS with Fastify, the implementation would go in the GET handler:

1. Check request.headers.range
2. Use fs.createReadStream(path, { start, end }) for partial reads
3. Set appropriate response headers and status 206