feat: async PDF cache i API media putanje za putne naloge

Uvodi background request tok za generiranje PDF-a s 24h cache-om i cleanup taskom preko Celery beat-a. Dodaje notification metadata download_url za ponovno preuzimanje bez regeneracije, API image endpointe za putne naloge i račune, te frontend integraciju za async download i produkcijski prikaz slika.

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

frontend/src/components/notifications/NotificationDetailModal.jsx

@@ -1,6 +1,11 @@
 import { useMemo } from 'preact/hooks';
 import ModalShell from '../ui/ModalShell';
-import { downloadWorkOrderInvoicesPdf, downloadWorkOrderPdf, downloadWorkOrderServiceRecordsPdf } from '../../stores/fleetDashboardStore';
+import {
+    downloadGeneratedPdfByUrl,
+    downloadWorkOrderInvoicesPdf,
+    downloadWorkOrderPdf,
+    downloadWorkOrderServiceRecordsPdf,
+} from '../../stores/fleetDashboardStore';
 
 function formatTimestamp(value) {
     if (!value) return '';
@@ -57,6 +62,12 @@ function getEntityAction(notification, meta) {
                     ? 'Preuzmi PDF servisnih zapisa'
                     : 'Preuzmi PDF putnog naloga',
             run() {
+                if (metadata.download_url) {
+                    return downloadGeneratedPdfByUrl(
+                        metadata.download_url,
+                        metadata.filename || `${metadata.work_order_id}.${metadata.pdf_type || 'work-order'}.pdf`
+                    );
+                }
                 if (metadata.pdf_type === 'invoices') {
                     return downloadWorkOrderInvoicesPdf(metadata.work_order_id);
                 }