feat(recommendation): SuggestArtists service for M5c

Add per-user artist-suggestion service ranking out-of-library MBIDs by
signal x similarity. Single-CTE SQL collects user likes (5x weight) and
recency-decayed plays, joins against artist_similarity_unmatched, and
filters in-library candidates plus non-terminal lidarr_requests. The
service resolves top-3 attribution seeds to artist names in a batched
GetArtistsByIDs call so the UI can render "because you liked X" reasons.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/db/queries/artists.sql

@@ -37,3 +37,8 @@ SELECT COUNT(*) FROM artists;
 
 -- name: CountArtistsMatching :one
 SELECT COUNT(*) FROM artists WHERE name ILIKE '%' || $1::text || '%';
+
+-- name: GetArtistsByIDs :many
+-- Batched lookup used by M5c suggestion attribution to resolve top-3
+-- contributing seed UUIDs back to artist names in one round-trip.
+SELECT * FROM artists WHERE id = ANY($1::uuid[]);