resolve_mirrored_playlist tried the mirrored-playlists primary key FIRST for
any all-digit ref. Deezer upstream ids are all-numeric, so a Deezer playlist id
was mistaken for the PK and the organize-by-playlist toggle resolved a wrong row
(or nothing) — the toggle silently wouldn't save / 'Open in Mirrored' missed.
Resolve by (source, source_playlist_id) first, fall back to PK only when the
source lookup misses. Thread the batch/wishlist source through the download-path
callers so numeric upstream ids resolve correctly there too. Spotify (base62
ids) is unaffected.
Seam tests: numeric Deezer id resolves by source (not PK), spotify alphanumeric
by source, PK fallback still works, profile-scoped, empty refs -> None.
