cfddns: make WEB_USER/WEB_PASSWORD form fields optional

Runtipi 'app update' validates required form_fields against its own
DB-tracked form state, not against app.env. Adding new required fields
to a pre-installed app blocks every update with 'Variable X is required'.
The manager binary already gracefully no-ops basic auth if either env
var is empty (logs a WARN). Drop required:true so updates pass; values
that exist in app.env keep working at runtime.

apps/cfddns/config.json

@@ -23,20 +23,20 @@
       "type": "text",
       "label": "Web UI Username",
       "env_variable": "CFDDNS_WEB_USER",
-      "required": true,
+      "required": false,
       "default": "admin",
       "min": 3,
       "max": 50,
-      "hint": "Username for HTTP basic auth on the management UI"
+      "hint": "Username for HTTP basic auth on the management UI. Leave blank to disable auth (the manager will log a WARN)."
     },
     {
       "type": "random",
       "label": "Web UI Password",
       "env_variable": "CFDDNS_WEB_PASSWORD",
-      "required": true,
+      "required": false,
       "min": 24,
       "encoding": "hex",
-      "hint": "Auto-generated 24-char hex password for the UI. View it in the app's Settings tab and copy it to your browser when prompted."
+      "hint": "Auto-generated on first install. View in the app's Settings tab. Leave blank to disable auth (manager logs a WARN)."
     },
     {
       "type": "password",