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/response_envelope.go

@@ -0,0 +1,12 @@
+package api
+
+// ResponseEnvelope is the standard wrapper for EZ-API JSON responses.
+// Code is a stable business code string (e.g., ok, invalid_request, not_found).
+// Message is empty for success and mirrors the top-level error for failures.
+// Data holds the original response payload.
+// swagger:model ResponseEnvelope
+type ResponseEnvelope struct {
+	Code    string `json:"code" example:"ok"`
+	Data    any    `json:"data" swaggertype:"object"`
+	Message string `json:"message" example:""`
+}