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>

backend/core/settings/base.py

@@ -1,5 +1,6 @@
 import os
 from pathlib import Path
+from celery.schedules import crontab
 from datetime import timedelta
 
 BASE_DIR = Path(__file__).resolve().parent.parent.parent
@@ -169,6 +170,12 @@ CACHES = {
 
 CELERY_ACCEPT_CONTENT = ['json']
 CELERY_TASK_SERIALIZER = 'json'
+CELERY_BEAT_SCHEDULE = {
+    'cleanup-expired-generated-pdfs-hourly': {
+        'task': 'modules.fleet.tasks.cleanup_expired_generated_pdfs_task',
+        'schedule': crontab(minute=0),
+    },
+}
 
 # URL kojim će preglednik dohvaćati statiku
 STATIC_URL = '/static/'