fix(soundcloud): handle preview-only tracks with source fallback

[AhmedMondo]

Changes

• Updated SoundCloud playback resolution to use the original SoundCloud permalink through api-v2.soundcloud.com/resolve, matching Lavaplayer's public web API flow instead of resolving api.soundcloud.com/tracks/{id}.
• Added support for SoundCloud's full_duration, monetization_model, policy, access, and snippet metadata so preview-only tracks can be distinguished from normally playable uploads.
• Added detection for SUB_HIGH_TIER, SNIP, /preview/ transcoding URLs, and resolved 30-second /playlist/0/30/ streams.
• Updated track duration handling to prefer full_duration over the 30-second preview duration.
• Updated SoundCloud transcoding selection to follow Lavaplayer-style format preference while rejecting known preview streams instead of attempting to play them as full tracks.
• Improved SoundCloud client_id handling by using browser-style request headers, refreshing the web client ID after 401 responses, and reducing the cached client ID lifetime to avoid stale credentials.
• Added the internal SOUNDCLOUD_PREVIEW_ONLY failure cause so preview-only playback failures can be handled separately from normal SoundCloud errors.
• Added configurable fallbackSources for preview-only SoundCloud tracks. The fallback flow searches by ISRC first, then artist + title, uses the existing getBestMatch() logic, and resolves the matched track through the fallback source.
• Added support for returning the fallback track through newTrack, allowing the existing player pipeline to fetch and play the replacement source without changing normal SoundCloud playback behavior.
• Normal SoundCloud tracks with full public transcodings continue to play directly from SoundCloud; fallback is only triggered when SoundCloud does not provide a full playable stream.

Why

• Some SoundCloud tracks, especially licensed tracks using SUB_HIGH_TIER with policy: SNIP, expose only 30-second preview transcodings through the public SoundCloud web API.
• These tracks were previously not playable in NodeLink. They would either resolve to a 30-second preview stream or fail playback with errors such as Track only has preview URL.
• SoundCloud-exclusive and normally playable uploads already work correctly and should continue using SoundCloud directly, so fallback must only be used for tracks where SoundCloud explicitly provides preview-only playback.
• The fallback mechanism allows NodeLink to recover these otherwise unplayable tracks through another configured source while keeping the originally requested SoundCloud track metadata and normal playback flow intact.
• The updated resolver also exposes the real track duration through full_duration. For example, a track may report duration: 30000 while its actual full_duration is several minutes.

Checkmarks

• I have tested the changes locally with both normal SoundCloud uploads and preview-only SUB_HIGH_TIER / SNIP tracks.
• I have tested a SoundCloud-exclusive track and confirmed it continues to play directly from SoundCloud without triggering fallback.
• I have tested a preview-only track and confirmed NodeLink detects the preview restriction and successfully plays a matched track through a configured fallback source.
• I have verified the fallback uses the existing source manager and getBestMatch() flow instead of introducing source-specific playback logic.
• I have tested the changes on a real Discord bot.

Additional info

The fallback is intentionally limited to SoundCloud tracks that resolve as preview-only. It is not used for normally playable SoundCloud tracks.

An example preview-only response from SoundCloud looks like:

duration: 30000
full_duration: 219350
monetization_model: SUB_HIGH_TIER
policy: SNIP

with only:

/preview/hls
/preview/progressive

available as transcodings.

Previously, these tracks could not be played successfully by NodeLink. With this change, NodeLink reports SOUNDCLOUD_PREVIEW_ONLY, searches the configured fallback sources, selects the closest match using existing matching logic, and continues playback through that source.

The fallback source order can be configured through:

soundcloud: {
  enabled: true,
  clientId: '',
  fallbackSources: ['youtube', 'deezer', 'jiosaavn', 'audiomack']
}

[github-actions]

All Contributors have signed the CLA. The PR is now allowed to be merged.
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[AhmedMondo]

I have read the CLA Document and I hereby sign the CLA