feat: add DNS credential management with Web UI

- New config/credential_store.go: persistent credential storage (credentials.json)
- New API: CRUD for DNS credentials + credential-types endpoint
- Certificate model now supports credential_id reference
- Create certificate auto-resolves credential_id to DNS config
- New Vue page: Credentials.vue for managing saved DNS keys
- CertCreate.vue: select existing credential or manual input + save as new
- Secrets masked in API responses, never exposed in list

frontend/src/router/index.ts

@@ -3,6 +3,7 @@ import Dashboard from '../views/Dashboard.vue'
 import CertList from '../views/CertList.vue'
 import CertCreate from '../views/CertCreate.vue'
 import NotifyConfig from '../views/NotifyConfig.vue'
+import Credentials from '../views/Credentials.vue'
 import Login from '../views/Login.vue'
 import { isLoggedIn } from '../api'
 
@@ -39,6 +40,12 @@ const router = createRouter({
       component: NotifyConfig,
       meta: { requiresAuth: true },
     },
+    {
+      path: '/credentials',
+      name: 'Credentials',
+      component: Credentials,
+      meta: { requiresAuth: true },
+    },
   ],
 })