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>

docker-compose.prod.yml

@@ -46,6 +46,26 @@ services:
       redis:
         condition: service_started
 
+  beat:
+    build:
+      context: ./backend
+      dockerfile: Dockerfile
+    container_name: 004erpbeat
+    restart: unless-stopped
+    env_file:
+      - .env
+    environment:
+      DEBUG: "False"
+      DJANGO_SETTINGS_MODULE: core.settings.production
+    command: celery -A core beat --loglevel=info
+    volumes:
+      - media_volume:/app/media
+    depends_on:
+      redis:
+        condition: service_started
+      worker:
+        condition: service_started
+
   flower:
     build:
       context: ./backend