refactor(P2): Handler 错误码精确化 — service 层引入哨兵错误，handler 层用 ClassifyHTTPStatus 分类

backend/internal/http/errors.go

@@ -0,0 +1,28 @@
+package http
+
+import (
+	"errors"
+	"net/http"
+
+	"gorm.io/gorm"
+)
+
+var (
+	ErrUnauthorized = errors.New("unauthorized")
+	ErrValidation   = errors.New("validation error")
+)
+
+func ClassifyHTTPStatus(err error) int {
+	switch {
+	case err == nil:
+		return http.StatusOK
+	case errors.Is(err, ErrUnauthorized):
+		return http.StatusUnauthorized
+	case errors.Is(err, ErrValidation):
+		return http.StatusBadRequest
+	case errors.Is(err, gorm.ErrRecordNotFound):
+		return http.StatusNotFound
+	default:
+		return http.StatusInternalServerError
+	}
+}