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/certificate.go

@@ -13,6 +13,7 @@ type Certificate struct {
 	ChallengeType string `json:"challenge_type"` // http, dns
 	DNSProvider   string `json:"dns_provider,omitempty"` // alidns, cloudflare, etc.
 	DNSConfig     string `json:"dns_config,omitempty"`  // JSON config for DNS provider
+	CredentialID  *uint  `json:"credential_id,omitempty"` // references saved credential
 
 	Status        string    `json:"status"` // pending, active, expired, error
 	CertURL       string    `json:"cert_url,omitempty"`