feat: add external song command support for AutoDJ (#16)

AutoDJ instances can now be configured with an external script/command
that returns the next song path on stdout. Priority order:
1. Manual queue (play-next)
2. External song command (if configured)
3. Built-in playlist (fallback if command fails)

Security: command runs via sh -c as same user, 5s default timeout
(configurable up to 30s), output validated as existing audio file.

Admin UI: song command field in AutoDJ create/edit modal with timeout
setting, CMD badge on instance cards.

Closes #16

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: syso