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

.gitignore

@@ -6,8 +6,6 @@ backend/data/
 
 # 编译产物
 backend/autossl
-# 预编译二进制（手动管理）
-backend/bin/
 node_modules/
 
 # IDE