"feat(api-server): add /api/v1/queue/next endpoint

[amontariol]

Description

Adds a new API endpoint /api/v1/queue/next to get information about the next song in the queue.

Motivation

Closes #3614

This addresses the need for applications to preload the next song without parsing the entire queue response. The current /api/v1/queue endpoint returns a large amount of data, and extracting just the next song requires complex parsing.

Changes

• Added new GET /api/v1/queue/next endpoint
• Returns next song information (title, videoId, thumbnail, etc.)
• Returns HTTP 204 when there's no next song
• Handles edge cases (empty queue, last song, etc.)

Testing

• ✅ Tested with active queue (returns next song data)
• ✅ Tested with last song in queue (returns 204)
• ✅ Tested with empty queue (returns 204)
• ✅ Verified response format matches API conventions

Example Usage

# Get next song
curl http://localhost:9863/api/v1/queue/next

# Example response
{
  "title": "Song Title",
  "videoId": "abc123",
  "thumbnail": {...},
  "lengthText": {...},
  "shortBylineText": {...}
}