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/main.go

@@ -53,6 +53,7 @@ func main() {
 	api.Use(authMiddleware())
 	{
 		certHandler := handlers.NewCertHandler(cfg)
+		credHandler := handlers.NewCredentialHandler()
 
 		// Certificate management
 		api.GET("/certificates", certHandler.ListCertificates)
@@ -69,6 +70,14 @@ func main() {
 		api.POST("/notify/test-feishu", handleTestFeishu)
 		api.POST("/notify/test-email", handleTestEmail)
 
+		// Credential management
+		api.GET("/credentials", credHandler.ListCredentials)
+		api.GET("/credentials/:id", credHandler.GetCredential)
+		api.POST("/credentials", credHandler.CreateCredential)
+		api.PUT("/credentials/:id", credHandler.UpdateCredential)
+		api.DELETE("/credentials/:id", credHandler.DeleteCredential)
+		api.GET("/credential-types", credHandler.GetCredentialTypes)
+
 		// Utility
 		api.GET("/renewals/check", certHandler.CheckRenewals)
 		api.GET("/stats", certHandler.Stats)