Ninth cluster: update_<source>_playlist_phase for the five sources sharing the
identical validation + full-message response (Tidal, Deezer, Qobuz,
Spotify-Public, YouTube) -> core.discovery.endpoints.update_playlist_phase(...),
wired via _update_source_playlist_phase + the _PHASE_LIST/_PHASE_LIST_YT
constants.
Per-source params:
- valid_phases — YouTube additionally allows 'parsed'.
- apply_extra_fields — Deezer/Qobuz/Spotify-Public also persist
download_process_id / converted_spotify_playlist_id from the body; Tidal and
YouTube do NOT, so they pass False (kept strictly 1:1 — the generic won't
apply those keys for them even if a caller sent them).
- not_found_message / error_label; get_json invoked inside the try.
NOT folded in: iTunes-Link — uses data.get('phase') (no "Phase not provided"
400) and returns a no-message payload.
Tests: +7 (404, missing-phase 400, invalid 400, happy path with extra-fields
suppressed, extra-fields applied when enabled, YouTube 'parsed' allowed,
exception -> 500). Full discovery suite: 205 passed.
web_server.py: -123 lines.
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