feat: advanced split — per-instrument stems + MIDI via /api/advanced_split

[francialisomlimoeiro]

Adds POST /api/advanced_split — Suno Studio "Extract Stems and MIDI" (Advanced split tab).

What it does

• Per-instrument stem extraction or removal — instrument names match the browser dropdown exactly (e.g. "Drum Kit", "Lead Vocal")
• MIDI note-events per instrument via the existing GET /api/gen/{id}/midi

Request contract

{
  "song_id": "<source clip id>",
  "instruments": ["Drum Kit", "Lead Vocal"],
  "mode": "extract",
  "include_midi": true
}

mode: "extract" (isolate) or "remove" (karaoke). include_midi also requests the MIDI batch and returns note-events.

Implementation notes

• Backed by the v2-web gen_stem task: one POST /api/generate/v2-web/ per instrument, stem_type_id=91 (constant for advanced mode; the instrument is carried by stem_type_group_name/stem_name), source clip in continue_clip_id
• Desktop-browser fingerprint headers on all v2-web/feed/midi calls (the Android client default headers are rejected upstream with permission_denied)
• metadata.user_tier sourced from GET /api/billing/info/, device-id header included
• Captcha token obtained through the existing getCaptcha() flow and passed in the payload when upstream demands it
• Audio stems polled with GET /api/feed/?ids= until complete; MIDI clip keeps polling on 400 Clip must be complete until it resolves within the deadline, then note-events are fetched from the midi endpoint

Link-first: all existing routes are untouched; this is purely additive (new endpoint + supporting SunoApi methods, docs + swagger entry).

Verified against a live account: 2 instruments in one call, 8 clips complete (extract/remove x audio/MIDI), audio downloadable from CDN, MIDI note-events non-empty.