feat(lyra)!: 接入分类查询并集中组装依赖

- 使用 Squirrel 与 sqlx 实现分类查询
- 集中组装依赖并向 handler 注入 model 接口
- 显式转换分类响应 DTO，统一空列表输出
- 补充路由与 handler 测试，同步架构文档

BREAKING CHANGE: 将 GET /hello 替换为 GET /test，响应由问候字符串改为分类列表

go/projects/lyra/internal/handler/test.go

@@ -0,0 +1,43 @@
+package handler
+
+import (
+	"lyra/internal/api"
+	"lyra/internal/httpx"
+	"lyra/internal/model"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+type TestHandler struct {
+	categoryModel model.CategoryModel
+}
+
+func NewTestHandler(categoryModel model.CategoryModel) *TestHandler {
+	return &TestHandler{categoryModel: categoryModel}
+}
+
+func (h *TestHandler) Hello() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		categories, err := h.categoryModel.FindAll(c.Request.Context())
+		if err != nil {
+			httpx.Fail(c, http.StatusInternalServerError, api.CodeInternalError, "获取分类列表失败")
+			return
+		}
+		httpx.OK(c, toCategoryResponses(categories))
+	}
+}
+
+// toCategoryResponses 显式把内部模型转换为响应 DTO；make 保证空结果输出 data:[] 而非 null。
+func toCategoryResponses(categories []*model.Category) []api.CategoryResponse {
+	responses := make([]api.CategoryResponse, 0, len(categories))
+	for _, category := range categories {
+		responses = append(responses, api.CategoryResponse{
+			ID:    category.ID,
+			Slug:  category.Slug,
+			Title: category.Title,
+			Sort:  category.Sort,
+		})
+	}
+	return responses
+}