Seventh cluster: start_<source>_sync for the five sources with the identical
flow (Tidal, Deezer, Qobuz, Spotify-Public, YouTube) ->
core.discovery.endpoints.start_sync(...), wired via _start_source_sync.
Validates phase, converts discovery results, seeds sync state, posts a
"... Sync Started" activity item, and submits to the sync executor. Per-source
pieces are params:
- sync_id_prefix (f"{prefix}_{key}"), not_found/not_ready messages, convert_fn.
- name/image accessors: Tidal reads an object (playlist_name_obj/
playlist_image_obj), the rest a dict (playlist_name_strict/playlist_image_dict).
- activity_label vs error_label DIFFER for Spotify-Public ("Spotify Link
Sync Started" activity, "Spotify Public" logs).
- submit_sync_task glue (_submit_sync_task) closes over sync_executor /
_run_sync_task / get_current_profile_id so the helper stays global-free.
NOT folded in: iTunes-Link (no final info log), ListenBrainz (submits the
task WITHOUT a playlist_image_url arg), Beatport (extra debug logging, chart).
Tests: +6 (404, not-ready 400, no-matches 400, full happy path with
state/sync-infra/submit/activity assertions, resync phases allowed,
exception -> 500). Full discovery suite: 189 passed.
web_server.py: -172 lines.
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