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/services/category_service.go

@@ -0,0 +1,80 @@
+package services
+
+import (
+	"context"
+
+	"github.com/fish/language/backend/internal/models"
+	"github.com/fish/language/backend/internal/repository/queries"
+)
+
+// CategoryService handles business logic for categories.
+type CategoryService struct {
+	q *queries.Queries
+}
+
+// NewCategoryService creates a new CategoryService.
+func NewCategoryService(q *queries.Queries) *CategoryService {
+	return &CategoryService{q: q}
+}
+
+// List returns all categories ordered by sort_order.
+func (s *CategoryService) List(ctx context.Context) ([]models.Category, error) {
+	rows, err := s.q.ListCategories(ctx)
+	if err != nil {
+		return nil, err
+	}
+	out := make([]models.Category, len(rows))
+	for i, r := range rows {
+		out[i] = toCategory(r)
+	}
+	return out, nil
+}
+
+// Get returns a single category by id.
+func (s *CategoryService) Get(ctx context.Context, id int32) (models.Category, error) {
+	row, err := s.q.GetCategory(ctx, id)
+	if err != nil {
+		return models.Category{}, err
+	}
+	return toCategory(row), nil
+}
+
+// Create creates a new category.
+func (s *CategoryService) Create(ctx context.Context, req models.CreateCategoryRequest) (models.Category, error) {
+	row, err := s.q.CreateCategory(ctx, queries.CreateCategoryParams{
+		Name:      req.Name,
+		SortOrder: req.SortOrder,
+	})
+	if err != nil {
+		return models.Category{}, err
+	}
+	return toCategory(row), nil
+}
+
+// Update updates an existing category.
+func (s *CategoryService) Update(ctx context.Context, id int32, req models.UpdateCategoryRequest) (models.Category, error) {
+	row, err := s.q.UpdateCategory(ctx, queries.UpdateCategoryParams{
+		ID:        id,
+		Name:      req.Name,
+		SortOrder: req.SortOrder,
+	})
+	if err != nil {
+		return models.Category{}, err
+	}
+	return toCategory(row), nil
+}
+
+// Delete removes a category by id.
+func (s *CategoryService) Delete(ctx context.Context, id int32) error {
+	return s.q.DeleteCategory(ctx, id)
+}
+
+func toCategory(c queries.Category) models.Category {
+	return models.Category{
+		ID:        c.ID,
+		Name:      c.Name,
+		SortOrder: c.SortOrder,
+		CreatedAt: c.CreatedAt,
+		UpdatedAt: c.UpdatedAt,
+	}
+}