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.dev.yml

@@ -50,6 +50,22 @@ services:
       - redis
       - worker
 
+  beat:
+    container_name: 004erpbeat
+    build:
+      context: ./backend
+      dockerfile: Dockerfile
+    command: celery -A core beat --loglevel=info
+    user: "1000:1000"
+    volumes:
+      - ./backend:/app
+      - media_volume:/app/media
+    env_file:
+      - .env
+    depends_on:
+      - redis
+      - worker
+
 volumes:
   media_volume:
     external: true # Ako je definiran u glavnom file-u, ovo ga povezuje