First cluster of the per-source playlist-discovery deduplication. The
convert_<source>_results_to_spotify_tracks functions (Tidal, Deezer, Qobuz,
Spotify-Public, YouTube, ListenBrainz) plus the already-generic
_convert_link_results_to_spotify_tracks were byte-identical apart from the
source label used in their log line.
Lift the shared body into core/discovery/endpoints.py as
convert_results_to_spotify_tracks(results, source_label); the 7 web_server
functions become 1-line delegations (names/signatures unchanged, so all
callers and behavior are identical — 1:1).
Beatport is intentionally NOT folded in: its converter coerces artist
objects to strings and emits a different track shape (source field, album
dict), so it keeps its own implementation.
Tests: tests/discovery/test_discovery_endpoints.py (12) pin both input
shapes (manual spotify_data / auto spotify_track+found), optional
track/disc numbers, falsy-0 omission, field defaults, skip-on-neither,
order preservation, if/elif precedence, empty input.
web_server.py: -209 lines. Full discovery suite: 151 passed.
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