feat(api): align response envelope schema

Switch response envelope business code to numeric and make message
consistently present. Add trace_id and optional details, and remove the
duplicate DTO envelope definition. Improve middleware path exclusion
handling and add a time-based trace ID fallback if crypto RNG fails.

BREAKING CHANGE: response envelope `code` is now `int` (was `string`) and
`message` semantics/defaults changed; clients must update parsing.

internal/dto/response.go

@@ -1,60 +0,0 @@
-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"`
-}