feat(social): show friend presence and require online listening

services/api/internal/social/presence.go

@@ -0,0 +1,16 @@
+package social
+
+import (
+	"context"
+
+	"github.com/google/uuid"
+)
+
+// Existing authenticated social/listening polls serve as heartbeats, including
+// while playback is paused or music visibility is off. Coalesce frequent polls
+// to one write per account per ten seconds; abandoned clients expire in 45s.
+func (s *Store) markPresent(ctx context.Context, id uuid.UUID) error {
+	_, e := s.pool.Exec(ctx, `UPDATE users SET social_seen_at=now()
+ WHERE id=$1 AND (social_seen_at IS NULL OR social_seen_at<now()-interval '10 seconds')`, id)
+	return e
+}