New 'Link / ID' input on the Search page: paste a Spotify / Apple Music /
MusicBrainz / Deezer URL (or a bare ID) and it's looked up directly on the
owning source — no fuzzy search, no scoring.
- core/search/by_id.py: source-agnostic parser (URL domain/path or bare-ID
format -> source,kind,id; numeric IDs fan out, first hit wins) + per-source
get-by-id dispatch + adapters projecting each provider's dict onto the
standard album/track card shape.
- /api/enhanced-search/by-id: thin additive route over resolve_identifier.
- Frontend: dedicated input that adopts the resolved source as active and
renders through the existing dropdown + download/import flow.
Purely additive — existing files are insertion-only; the resolver runs only
behind the new route. 29 seam tests cover parsing, shaping, fan-out, and
not-found.
