fix: prevent resolveOrCreateReplicatedUser from recreating deleted user stubs

When a federated user's identity has been tombstoned (isDeleted=1),
findFederatedUser filters them out, causing resolveOrCreateReplicatedUser
to silently create a new stub — a "zombie" resurrection. This guard checks
for a deleted row before creating a stub and returns null instead.

All 11 call sites across federation.ts and dm.ts have been updated with
appropriate null guards: federation relay handlers reject or skip the event
(participant_not_found / accepted no-op), while dm.ts routes convert null
to undefined so the existing 404 path handles it.

packages/server/src/routes/dm.ts

@@ -384,7 +384,7 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
 
     if (homeUserId && homeInstance) {
       // Federated identity: resolve or create a replicated user stub
-      targetUser = resolveOrCreateReplicatedUser(homeUserId, homeInstance, db);
+      targetUser = resolveOrCreateReplicatedUser(homeUserId, homeInstance, db) ?? undefined;
     } else if (userId && typeof userId === 'string') {
       // Local ID: direct lookup (existing behavior)
       targetUser = db.select().from(schema.users).where(eq(schema.users.id, userId)).get();
@@ -570,7 +570,7 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
 
       if (identity.homeUserId && identity.homeInstance) {
         // Federated user — resolve via homeUserId, creating a replicated stub if needed
-        localUser = resolveOrCreateReplicatedUser(identity.homeUserId, identity.homeInstance, db);
+        localUser = resolveOrCreateReplicatedUser(identity.homeUserId, identity.homeInstance, db) ?? undefined;
       } else {
         // Local user — direct ID lookup
         localUser = db.select().from(schema.users).where(
@@ -865,7 +865,7 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
 
     if (homeUserId && homeInstance) {
       // Federated identity: resolve or create a replicated user stub
-      targetUser = resolveOrCreateReplicatedUser(homeUserId, homeInstance, db);
+      targetUser = resolveOrCreateReplicatedUser(homeUserId, homeInstance, db) ?? undefined;
     } else if (targetUserIdRaw && typeof targetUserIdRaw === 'string') {
       // Local ID: direct lookup (existing behavior)
       targetUser = db.select().from(schema.users).where(eq(schema.users.id, targetUserIdRaw)).get();