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

backend/config/config.go

@@ -76,6 +76,7 @@ func InitStore(cfg *Config) {
 func InitAll(cfg *Config) {
 	InitStore(cfg)
 	InitNotifyStore(cfg)
+	InitCredentialStore(cfg)
 }
 
 // Load reads certificates from JSON file into memory