fix: remove user_ids validation that blocked notes-to-self

The 'at least one user_id is required' check ran before the creator
was added to memberSet, so {user_ids: []} always 400'd. Now empty
user_ids is allowed — the creator is always in memberSet.

internal/dm/handlers.go

@@ -70,10 +70,7 @@ func (h *Handler) Create(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, `{"error":"invalid request"}`, http.StatusBadRequest)
 		return
 	}
-	if len(req.UserIDs) == 0 {
-		http.Error(w, `{"error":"at least one user_id is required"}`, http.StatusBadRequest)
-		return
-	}
+	// ponytail: user_ids can be empty for self-DM (notes to self)
 
 	// Build unique member set including creator.
 	memberSet := map[string]struct{}{userID: {}}