Feature: Referral-System — User wirbt User

- DB: referral_code (8-stellig, eindeutig) + referred_by zu users
  Bestehende User erhalten automatisch einen Code
- GET /api/auth/referral: Code, Link und Anzahl geworbener User
- POST /api/auth/register: ref_code Parameter für Zuordnung
- Settings: 'App empfehlen'-Karte mit Link, Teilen-Button und Botschafter-Badges
  (Botschafter ab 1, Super ab 5, Top ab 10 Einladungen)
- app.js: ?ref=CODE aus URL in sessionStorage speichern
- APP_VER 222, SW by-v244

backend/static/js/api.js

@@ -78,8 +78,10 @@ const API = (() => {
     login(email, password) {
       return post('/auth/login', { email, password });
     },
-    register(email, password, name) {
-      return post('/auth/register', { email, password, name });
+    register(email, password, name, ref_code) {
+      const body = { email, password, name };
+      if (ref_code) body.ref_code = ref_code;
+      return post('/auth/register', body);
     },
     logout() {
       localStorage.removeItem('by_token');
@@ -88,6 +90,7 @@ const API = (() => {
     me() {
       return get('/auth/me');
     },
+    referral: () => get('/auth/referral'),
   };
 
   // ----------------------------------------------------------