feat(server/playlists): Discover system playlist

Adds the third system playlist variant: 'discover'. Surfaces 100
tracks the operator has not played and not liked, biased toward
artists they rarely play (< 10 plays) and complemented by tracks
liked by other users plus a random unheard sample. Cold start
collapses to all-random; single-user servers redistribute the
cross-user-likes allocation equally across the other two buckets.

The build runs as a fourth phase inside BuildSystemPlaylists
alongside For You and the seed-artist mixes. Same daily-deterministic
md5(track_id || dateStr) ordering as the other variants. Per-album
(<=2) and per-artist (<=3) caps prevent collapse onto a single
prolific artist. Buckets interleave round-robin so the playlist
order doesn't front-load one bucket's flavour.

Post-commit collage generation (already wired) gives Discover the
same 4-cell cover treatment as the other system playlists — no
new collage code needed.

Tests cover the slot-redistribution table (all-available,
cold-start, single-user, partial-dormant, fully-empty), the
per-album/per-artist caps, the round-robin interleave, and a
DB-backed cold-start integration test that asserts a Discover
playlist lands with non-zero tracks.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/playlists/system_test.go

@@ -105,6 +105,8 @@ 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.
 		default:
 			t.Errorf("unknown system_variant=%q", *r.SystemVariant)
 		}
@@ -320,6 +322,57 @@ func TestListActiveUsersForSystemPlaylists(t *testing.T) {
 	}
 }
 
+func TestBuildSystemPlaylists_DiscoverColdStart(t *testing.T) {
+	// User has no plays / likes → Discover's dormant + cross-user
+	// buckets are empty (or minimal), all slots come from random unheard.
+	pool := newPool(t)
+	logger := discardLogger()
+	u, _ := seedActiveLibrary(t, pool, "discover_cold", 4, 5)
+	// seedActiveLibrary seeds 3 plays per track; delete them so dormant
+	// bucket (< 10 plays per artist) and random bucket both operate on
+	// a true cold-start user with no prior play history.
+	if _, err := pool.Exec(context.Background(),
+		"DELETE FROM play_events WHERE user_id = $1", u.ID); err != nil {
+		t.Fatalf("clear plays: %v", err)
+	}
+
+	if err := playlists.BuildSystemPlaylists(context.Background(), pool, logger, u.ID, time.Now().UTC(), t.TempDir()); err != nil {
+		t.Fatalf("build: %v", err)
+	}
+
+	rows, err := dbq.New(pool).ListPlaylistsByUserAndKind(context.Background(), dbq.ListPlaylistsByUserAndKindParams{
+		UserID: u.ID, Column2: "system",
+	})
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	var foundDiscover bool
+	for _, r := range rows {
+		if r.SystemVariant != nil && *r.SystemVariant == "discover" {
+			foundDiscover = true
+			if r.TrackCount == 0 {
+				t.Errorf("Discover playlist has zero tracks on cold start (random bucket should fill)")
+			}
+			if r.TrackCount > 100 {
+				t.Errorf("Discover track_count = %d, want <= 100", r.TrackCount)
+			}
+		}
+	}
+	if !foundDiscover {
+		t.Errorf("expected a 'discover' system playlist; got variants: %v", systemVariantsOf(rows))
+	}
+}
+
+func systemVariantsOf(rows []dbq.ListPlaylistsByUserAndKindRow) []string {
+	out := make([]string, 0, len(rows))
+	for _, r := range rows {
+		if r.SystemVariant != nil {
+			out = append(out, *r.SystemVariant)
+		}
+	}
+	return out
+}
+
 func TestStartupRecoveryClearsStaleInFlight(t *testing.T) {
 	pool := newPool(t)
 	ctx := context.Background()