refactor(api): standardize DTOs and update swagger

Decouple API contract from internal models by introducing dedicated DTOs for requests and responses.
- Add Response DTOs for all resources (API Keys, Bindings, Models, Namespaces, etc.)
- Update Swagger annotations to use DTOs with field examples instead of internal models
- Refactor handlers to bind and return DTO structures
- Consolidate request/response definitions in the dto package

internal/dto/namespace.go

@@ -0,0 +1,22 @@
+package dto
+
+import "time"
+
+// NamespaceResponse represents a namespace in API responses.
+// @Description Namespace response
+type NamespaceResponse struct {
+	ID          uint      `json:"id" example:"1"`
+	CreatedAt   time.Time `json:"created_at"`
+	UpdatedAt   time.Time `json:"updated_at"`
+	Name        string    `json:"name" example:"default"`
+	Status      string    `json:"status" example:"active"`
+	Description string    `json:"description,omitempty" example:"Default namespace"`
+}
+
+// NamespaceDTO defines inbound payload for namespace creation/update.
+// @Description Namespace create/update request
+type NamespaceDTO struct {
+	Name        string `json:"name" example:"default"`
+	Status      string `json:"status" example:"active"`
+	Description string `json:"description,omitempty" example:"Default namespace"`
+}