feat(similarity): persist unmatched similar-artist MBIDs for M5c

upsertArtistSimilar keeps the existing matched path (top-K rows into
artist_similarity) and adds a parallel unmatched-persist loop with the
same top-K cap. Empty-name rows are skipped — we can't render a
suggestion card without a name. Logs unmatched-side errors at WARN
without aborting the tick (mirrors the matched-path policy).

internal/similarity/worker.go

@@ -189,9 +189,10 @@ func (w *Worker) upsertArtistSimilar(ctx context.Context, q *dbq.Queries, artist
 		}
 	}
 
-	taken := 0
+	// Matched: in-library similars → artist_similarity (existing path).
+	takenMatched := 0
 	for _, r := range results {
-		if taken >= w.topK {
+		if takenMatched >= w.topK {
 			break
 		}
 		localID, ok := idByMBID[r.MBID]
@@ -207,6 +208,34 @@ func (w *Worker) upsertArtistSimilar(ctx context.Context, q *dbq.Queries, artist
 			w.logger.Warn("similarity: UpsertArtistSimilarity", "err", uerr)
 			continue
 		}
-		taken++
+		takenMatched++
+	}
+
+	// Unmatched: out-of-library similars → artist_similarity_unmatched (M5c).
+	// Same top-K cap as the matched path. Skip rows missing a name — we can't
+	// render a suggestion card without one.
+	takenUnmatched := 0
+	for _, r := range results {
+		if takenUnmatched >= w.topK {
+			break
+		}
+		if _, inLib := idByMBID[r.MBID]; inLib {
+			continue
+		}
+		if r.Name == "" {
+			w.logger.Debug("similarity: skipping unmatched similar with empty name", "mbid", r.MBID)
+			continue
+		}
+		if uerr := q.UpsertArtistSimilarityUnmatched(ctx, dbq.UpsertArtistSimilarityUnmatchedParams{
+			SeedArtistID:  artistAID,
+			CandidateMbid: r.MBID,
+			CandidateName: r.Name,
+			Score:         r.Score,
+			Source:        "listenbrainz",
+		}); uerr != nil {
+			w.logger.Warn("similarity: UpsertArtistSimilarityUnmatched", "err", uerr)
+			continue
+		}
+		takenUnmatched++
 	}
 }