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/dashboard/WorkOrderInvoicesPdfPage.jsx

@@ -143,8 +143,9 @@ export default function WorkOrderInvoicesPdfPage() {
         }
         if (Array.isArray(invoices)) {
             invoices.forEach((invoice) => {
-                if (invoice?.image) {
-                    paths.push(invoice.image);
+                const invoiceImage = invoice?.image_url || invoice?.image;
+                if (invoiceImage) {
+                    paths.push(invoiceImage);
                 }
             });
         }
@@ -304,7 +305,7 @@ export default function WorkOrderInvoicesPdfPage() {
                                         <div><span className="font-semibold">opis:</span> {invoice.opis || '-'}</div>
                                     </div>
                                     <InvoiceImagePreview
-                                        imageUrl={getAuthenticatedMediaSrc(invoice.image) || resolveMediaUrl(invoice.image)}
+                                        imageUrl={getAuthenticatedMediaSrc(invoice.image_url || invoice.image) || resolveMediaUrl(invoice.image_url || invoice.image)}
                                         alt={`Račun ${invoice.naziv_racuna}`}
                                     />
                                 </div>