test: correct the A regression + finish B residual (coverart boot, stale system_test)

A redo: the prior commit truncated cover_art_sources_meta, but
SettingsService.reconcile() only READS that singleton (seeded once by
migration 0018) and never recreates it → ~45 coverart/api tests hard-
failed "get current sources version: no rows". Correct reset: keep
cover_art_provider_settings in the truncate set (boot idempotently
re-UpsertProviderSettings), drop cover_art_sources_meta from it, and
instead `UPDATE cover_art_sources_meta SET current_version = 1` so the
row survives while cross-test version accumulation is cleared.

B residual (exposed once the play_events FK fix let these run):
- seedQuarantine used reason 'test-hide', invalid for the
  lidarr_quarantine_reason enum (bad_rip/wrong_file/wrong_tags/
  duplicate/other) → use 'other'.
- TestBuildSystemPlaylists_SufficientActivity predated #411/#352: the
  variant switch errored on the 5 new seedless mixes and capped
  track_count at 25. Accept deep_cuts/rediscover/new_for_you/
  on_this_day/first_listens as seedless; raise the cap to 100.

Test/test-harness only.

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

internal/playlists/system_test.go

@@ -45,7 +45,7 @@ func seedQuarantine(t *testing.T, pool *pgxpool.Pool, userID, trackID pgtype.UUI
 	t.Helper()
 	_, err := pool.Exec(context.Background(), `
 		INSERT INTO lidarr_quarantine (user_id, track_id, reason)
-		VALUES ($1, $2, 'test-hide')
+		VALUES ($1, $2, 'other')
 	`, userID, trackID)
 	if err != nil {
 		t.Fatalf("seed quarantine: %v", err)
@@ -112,16 +112,19 @@ func TestBuildSystemPlaylists_SufficientActivity(t *testing.T) {
 			if !r.SeedArtistID.Valid {
 				t.Errorf("songs_like_artist row should have non-NULL seed_artist_id")
 			}
-		case "discover":
-			// Discover playlist is valid — no seed_artist_id required.
+		case "discover", "deep_cuts", "rediscover", "new_for_you", "on_this_day", "first_listens":
+			// Discover + the #411 discovery mixes are all seedless —
+			// no seed_artist_id required.
 		default:
 			t.Errorf("unknown system_variant=%q", *r.SystemVariant)
 		}
 		if r.TrackCount == 0 {
 			t.Errorf("row %s: empty track_count", uuidString(r.ID))
 		}
-		if r.TrackCount > 25 {
-			t.Errorf("row %s: track_count=%d exceeds 25", uuidString(r.ID), r.TrackCount)
+		// For-You / Discover / the discovery mixes are sized to ~100
+		// (#352/#411); songs_like_artist stays at systemMixLength (25).
+		if r.TrackCount > 100 {
+			t.Errorf("row %s: track_count=%d exceeds 100", uuidString(r.ID), r.TrackCount)
 		}
 	}
 	if !hasForYou {