fix: crontab validation

src/restic_compose_backup/config.py

@@ -3,7 +3,7 @@ import os
 
 class Config:
     default_backup_command = ". /env.sh && /backup.sh && rcb cleanup > /proc/1/fd/1 2>&1"
-    default_crontab_schedule = "0 2 * * *"
+    default_crontab_schedule = "0 */4 * * *"
 
     """Bag for config values"""
     def __init__(self, check=True):

src/restic_compose_backup/cron.py

@@ -34,10 +34,6 @@ def validate_schedule(schedule: str):
     if len(parts) != 5:
         return False
 
-    for p in parts:
-        if p != '*' and not p.isdigit():
-            return False
-
     minute, hour, day, month, weekday = parts
     try:
         validate_field(minute, 0, 59)