Clean expired API keys every 24h

If someone logs in, then never logs back in while the key expires, then the key is stored in the dict and never deleted. So every 24 hours delete all keys that have been expired for more than 24 hours. Avoids a memory leak-like situation.

backend/base/definitions.py

@@ -55,6 +55,7 @@ class Constants:
     ADMIN_API_EXTENSION = "/admin"
     ADMIN_PREFIX = API_PREFIX + ADMIN_API_EXTENSION
     API_KEY_LENGTH = 32 # hexadecimal characters
+    API_KEY_CLEANUP_INTERVAL = 86400 # seconds
 
     DB_FOLDER = ("db",)
     DB_NAME = "MIND.db"