fix: zamijeni auth polling health endpointom

Dodaje backend /api/health/ endpoint i prebacuje frontend heartbeat sa /api/token/ OPTIONS na /api/health/ GET kako bi se izbjeglo periodicko gadanje auth rute.

Dodaje TTL cache za clientStore (lastFetchedAt + force opcija) kako bi se smanjili redundantni crm/clients pozivi pri navigaciji.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/core/urls.py

@@ -2,14 +2,19 @@
 from django.conf import settings
 from django.conf.urls.static import static
 from django.contrib import admin
+from django.http import JsonResponse
 from django.urls import path, include
 from rest_framework_simplejwt.views import (
     TokenObtainPairView,
     TokenRefreshView,
 )
 
+def api_health(_request):
+    return JsonResponse({'status': 'ok'})
+
 urlpatterns = [
     path('admin/', admin.site.urls),
+    path('api/health/', api_health, name='api_health'),
     
     # Uključujemo rute iz modula (svaki modul ima svoj urls.py)
     path('api/crm/', include('modules.crm.urls')),