fix(social): tighten comment on federated-stub filter

Drop the file-path reference per code-review suggestion (paths in
comments rot); keep the load-bearing why — substring of the domain
matches every stub because stubs are stored as <homeUserId>@<domain>.

packages/server/src/routes/social.ts

@@ -689,12 +689,10 @@ export async function socialRoutes(app: FastifyInstance): Promise<void> {
       eq(schema.users.isDeleted, 0),
       eq(schema.users.discoverable, 1),
       ne(schema.users.id, request.userId),
-      // Exclude replicated federated stubs — federated users are surfaced
-      // via the client-side cross-instance fan-out in
-      // packages/web/src/stores/socialStore.ts (searchUsers), which dedupes
-      // by canonical identity. Returning stubs here would be a noisy
-      // duplicate source AND would leak domain-suffix substring matches
-      // (stubs are stored as <homeUserId>@<domain>).
+      // Exclude replicated federated stubs: their stored username is
+      // `<homeUserId>@<domain>`, so a substring of the domain would match
+      // every stub from that instance. Federated users are surfaced via
+      // the client-side cross-instance fan-out instead.
       sql`(${schema.users.homeInstance} IS NULL OR ${schema.users.homeInstance} = '')`,
       or(
         like(schema.users.username, pattern),