新增语言学习后台服务

- 分类管理：支持分类的增删改查
- 单词管理：支持单词的增删改查，包含音标、释义、例句等字段
- 更新 README 项目说明

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/handlers/response.go

@@ -0,0 +1,40 @@
+package handlers
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+
+	"github.com/jackc/pgx/v5"
+)
+
+// RespondJSON writes a JSON response with the given status code.
+func RespondJSON(w http.ResponseWriter, status int, data any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(data)
+}
+
+// RespondSuccess returns the common success envelope.
+func RespondSuccess(w http.ResponseWriter, data any) {
+	RespondJSON(w, http.StatusOK, modelsAPIResponse{Code: 0, Message: "ok", Data: data})
+}
+
+// RespondError returns the common error envelope.
+func RespondError(w http.ResponseWriter, status int, code int, message string) {
+	RespondJSON(w, status, modelsAPIResponse{Code: code, Message: message, Data: nil})
+}
+
+// MapError maps repository/ service errors to HTTP status and code.
+func MapError(err error) (status int, code int, message string) {
+	if errors.Is(err, pgx.ErrNoRows) {
+		return http.StatusNotFound, 10001, "resource not found"
+	}
+	return http.StatusInternalServerError, 10000, "internal server error"
+}
+
+type modelsAPIResponse struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+	Data    any    `json:"data"`
+}