Fix update_jail_config to ignore backend field

backend/app/services/config_service.py

@@ -351,8 +351,8 @@ async def update_jail_config(
         await _set("datepattern", update.date_pattern)
     if update.dns_mode is not None:
         await _set("usedns", update.dns_mode)
-    if update.backend is not None:
-        await _set("backend", update.backend)
+    # backend is managed by fail2ban and cannot be changed at runtime by API.
+    # This field is therefore ignored during updates.
     if update.log_encoding is not None:
         await _set("logencoding", update.log_encoding)
     if update.prefregex is not None: