Parse [database]max_queue_hours as integer, handle invalid format

lacre/keymgmt.py

@@ -5,6 +5,10 @@ from datetime import datetime, timedelta
 from lacre.config import get_item
 
 
+# By default, we let keys stay in confirmation queue for 1 hour.
+_DEFAULT_TTL = 1
+
+
 def calculate_expiry_date(now: datetime) -> datetime:
     """Calculate date-time of key queue item expiry.
 
@@ -12,5 +16,15 @@ def calculate_expiry_date(now: datetime) -> datetime:
     [database]max_queue_hours, return a date-time object that should be
     older than any key in our confirmation queue.  If a key is older
     than this threshold, we should remove it."""
-    max_hours = get_item('database', 'max_queue_hours', 1)
+    max_hours = _get_ttl()
     return now - timedelta(hours=max_hours)
+
+
+def _get_ttl():
+    max_hours = get_item('database', 'max_queue_hours', _DEFAULT_TTL)
+    try:
+        ttl =  int(max_hours)
+        return ttl
+    except ValueError:
+        # Not a valid integer, so we return the default.
+        return _DEFAULT_TTL