feat(api): wrap JSON responses in envelope

Add response envelope middleware to standardize JSON responses as
`{code,data,message}` with consistent business codes across endpoints.
Update Swagger annotations and tests to reflect the new response shape.

BREAKING CHANGE: API responses are now wrapped in a response envelope; clients must read payloads from `data` and handle `code`/`message` fields.

internal/api/status_handler.go

@@ -47,7 +47,7 @@ type AboutResponse struct {
 // @Description  Returns public runtime status information without sensitive data
 // @Tags         Public
 // @Produce      json
-// @Success      200  {object}  StatusResponse
+// @Success      200  {object}  ResponseEnvelope{data=StatusResponse}
 // @Router       /status [get]
 func (h *StatusHandler) Status(c *gin.Context) {
 	// Check health status
@@ -67,7 +67,7 @@ func (h *StatusHandler) Status(c *gin.Context) {
 // @Description  Returns system metadata for display on an about page
 // @Tags         Public
 // @Produce      json
-// @Success      200  {object}  AboutResponse
+// @Success      200  {object}  ResponseEnvelope{data=AboutResponse}
 // @Router       /about [get]
 func (h *StatusHandler) About(c *gin.Context) {
 	resp := AboutResponse{