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/common.go

@@ -0,0 +1,7 @@
+package dto
+
+// DeleteResponse represents a successful delete operation response.
+// @Description Delete operation response
+type DeleteResponse struct {
+	Status string `json:"status" example:"deleted"`
+}