Tombstoned users, replicated federated stubs, and users with
discoverable=0 were all surfacing in Add Friend search results.
Add the three WHERE filters that /api/social/discover already
applies. Federated users continue to be surfaced via the
client-side cross-instance fan-out in socialStore.searchUsers.
New tests: social.test.ts covers all five filter cases plus
existing self-exclusion and displayName-match behaviours.
Jannis Braun
b37d3bfa29