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

services/api/migrations/00023_social_presence.sql

@@ -0,0 +1,7 @@
+-- +goose Up
+-- Reachability is independent of music-sharing privacy. Any active client keeps
+-- this lease alive; stopping one device cannot mark another device offline.
+ALTER TABLE users ADD COLUMN social_seen_at timestamptz;
+
+-- +goose Down
+ALTER TABLE users DROP COLUMN social_seen_at;