feat: introduce AP scheduler for polling vault

backend/letters/apps.py

@@ -1,5 +1,19 @@
+import os
+
 from django.apps import AppConfig
 
 
 class LettersConfig(AppConfig):
     name = "letters"
+
+    def ready(self):
+        """
+        Start the scheduler only when the server is starting.
+        NOTE: If we don't check for RUN_MAIN, the scheduler triggers for all django operations (migration, test etc.)
+        """
+
+        if not (os.environ.get("RUN_MAIN") == "true" or os.environ.get("WERKZEUG_RUN_MAIN") == "true"):
+            return
+        from .tasks import start_scheduler
+
+        start_scheduler()