Add default schedules to check_data and PDF expiry

aleksis/core/data_checks.py

 import logging
+from datetime import timedelta
 
 from django.apps import apps
 from django.contrib.contenttypes.models import ContentType
@@ -236,7 +237,7 @@ class DataCheckRegistry:
         return [(check.name, check.verbose_name) for check in cls.data_checks]
 
 
-@recorded_task
+@recorded_task(run_every=timedelta(minutes=15))
 def check_data(recorder: ProgressRecorder):
     """Execute all registered data checks and send email if activated."""
     for check in recorder.iterate(DataCheckRegistry.data_checks):

aleksis/core/util/pdf.py

 import os
 import subprocess  # noqa
+from datetime import timedelta
 from tempfile import TemporaryDirectory
 from typing import Optional, Tuple, Union
 from urllib.parse import urljoin
@@ -129,7 +130,7 @@ def clean_up_expired_pdf_files() -> None:
     PDFFile.objects.filter(expires_at__lt=timezone.now()).delete()
 
 
-@app.task
+@app.task(run_every=timedelta(days=1))
 def clean_up_expired_pdf_files_task() -> None:
     """Clean up expired PDF files."""
     return clean_up_expired_pdf_files()