feat(api): standardize response envelope behavior

Add shared response DTOs and enhance the response envelope middleware with
excluded paths, trace ID generation fallback, and automatic extraction of
error details from handler responses. Update default business code mapping
for 503 and 504, and adjust idempotency detection to only treat the new
envelope format as already-wrapped.

BREAKING CHANGE: responses using the old envelope format (e.g., string
`code`) are now wrapped into the new standard envelope.

internal/dto/response.go

@@ -0,0 +1,60 @@
+package dto
+
+// ResponseEnvelope is the standard API response wrapper.
+// All JSON API responses are wrapped in this envelope format.
+// @Description Standard API response envelope
+type ResponseEnvelope struct {
+	// Business code: 0 for success, non-zero for errors
+	// 0 = success
+	// 1xxx = common errors (invalid param, unauthorized, forbidden, rate limited)
+	// 4xxx = client errors (resource not found, conflict, invalid state)
+	// 5xxx = server errors (internal error, service unavailable, timeout)
+	Code int `json:"code" example:"0"`
+
+	// Human-readable message: "success" for success, error description for errors
+	Message string `json:"message" example:"success"`
+
+	// Response data: business payload for success, null for errors
+	Data any `json:"data"`
+
+	// Request trace ID for debugging and log correlation
+	TraceID string `json:"trace_id" example:"a1b2c3d4e5f6g7h8"`
+
+	// Optional error details (e.g., field validation errors)
+	Details any `json:"details,omitempty"`
+}
+
+// ErrorResponse is returned when an error occurs.
+// @Description Error response format
+type ErrorResponse struct {
+	// Business error code (non-zero)
+	Code int `json:"code" example:"4001"`
+
+	// Error message
+	Message string `json:"message" example:"resource not found"`
+
+	// Always null for errors
+	Data any `json:"data" example:"null"`
+
+	// Request trace ID
+	TraceID string `json:"trace_id" example:"a1b2c3d4e5f6g7h8"`
+
+	// Optional structured error details
+	Details any `json:"details,omitempty"`
+}
+
+// SuccessResponse is returned for successful operations.
+// @Description Success response format
+type SuccessResponse struct {
+	// Always 0 for success
+	Code int `json:"code" example:"0"`
+
+	// Always "success"
+	Message string `json:"message" example:"success"`
+
+	// Response payload
+	Data any `json:"data"`
+
+	// Request trace ID
+	TraceID string `json:"trace_id" example:"a1b2c3d4e5f6g7h8"`
+}