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_integration_test.go

@@ -394,3 +394,73 @@ func TestTickOnce_NoMBIDOnTrack_Skipped(t *testing.T) {
 		t.Errorf("no-MBID track produced rows: %d", got)
 	}
 }
+
+// TestUpsertArtistSimilar_PersistsUnmatchedToTable: M5c. Calls
+// upsertArtistSimilar directly with a mix of in-library and out-of-library
+// similar-artist payloads; verifies the matched path lands in
+// artist_similarity (1 row), the unmatched path lands in
+// artist_similarity_unmatched (3 rows), and rows with empty Name are
+// skipped (we can't render a suggestion without a name).
+func TestUpsertArtistSimilar_PersistsUnmatchedToTable(t *testing.T) {
+	f := newFixture(t)
+	ctx := context.Background()
+
+	// Seed one additional in-library artist that will be the in-library match.
+	inLibMBID := "in-lib-mbid-123"
+	inLibArtist, err := f.q.UpsertArtist(ctx, dbq.UpsertArtistParams{
+		Name: "InLib Artist", SortName: "InLib Artist", Mbid: &inLibMBID,
+	})
+	if err != nil {
+		t.Fatalf("seed in-lib artist: %v", err)
+	}
+
+	w := newTestWorker(f, "")
+
+	similars := []listenbrainz.SimilarArtist{
+		{MBID: inLibMBID, Name: "InLib Artist", Score: 0.95},
+		{MBID: "out-mbid-1", Name: "Outsider One", Score: 0.85},
+		{MBID: "out-mbid-2", Name: "Outsider Two", Score: 0.80},
+		{MBID: "out-mbid-3", Name: "Outsider Three", Score: 0.70},
+		{MBID: "out-mbid-4", Name: "", Score: 0.60}, // empty name — skipped
+	}
+	w.upsertArtistSimilar(ctx, f.q, f.artist.ID, similars)
+
+	// Matched path: 1 row in artist_similarity for the in-library candidate.
+	var matchedCount int
+	if err := f.pool.QueryRow(ctx,
+		"SELECT count(*) FROM artist_similarity WHERE artist_a_id = $1",
+		f.artist.ID,
+	).Scan(&matchedCount); err != nil {
+		t.Fatalf("count matched: %v", err)
+	}
+	if matchedCount != 1 {
+		t.Errorf("artist_similarity rows = %d, want 1 (only the in-library match)", matchedCount)
+	}
+
+	// Unmatched path: 3 rows (out-mbid-1/2/3); the empty-name row is skipped.
+	var unmatchedCount int
+	if err := f.pool.QueryRow(ctx,
+		"SELECT count(*) FROM artist_similarity_unmatched WHERE seed_artist_id = $1",
+		f.artist.ID,
+	).Scan(&unmatchedCount); err != nil {
+		t.Fatalf("count unmatched: %v", err)
+	}
+	if unmatchedCount != 3 {
+		t.Errorf("artist_similarity_unmatched rows = %d, want 3", unmatchedCount)
+	}
+
+	// Verify a specific row's name + score round-tripped correctly.
+	var name string
+	var score float64
+	if err := f.pool.QueryRow(ctx,
+		"SELECT candidate_name, score FROM artist_similarity_unmatched WHERE seed_artist_id = $1 AND candidate_mbid = $2",
+		f.artist.ID, "out-mbid-1",
+	).Scan(&name, &score); err != nil {
+		t.Fatalf("fetch out-mbid-1: %v", err)
+	}
+	if name != "Outsider One" || score != 0.85 {
+		t.Errorf("row = (%q, %v), want (Outsider One, 0.85)", name, score)
+	}
+
+	_ = inLibArtist
+}