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/word_service.go

@@ -0,0 +1,155 @@
+package services
+
+import (
+	"context"
+	"math"
+
+	"github.com/fish/language/backend/internal/models"
+	"github.com/fish/language/backend/internal/repository/queries"
+)
+
+const defaultPageSize = 20
+const maxPageSize = 100
+
+// WordService handles business logic for words.
+type WordService struct {
+	q *queries.Queries
+}
+
+// NewWordService creates a new WordService.
+func NewWordService(q *queries.Queries) *WordService {
+	return &WordService{q: q}
+}
+
+// List returns a paginated list of words with optional filters.
+func (s *WordService) List(ctx context.Context, req models.ListWordsRequest) (models.ListWordsResponse, error) {
+	page := req.Page
+	if page < 1 {
+		page = 1
+	}
+	pageSize := req.PageSize
+	if pageSize < 1 {
+		pageSize = defaultPageSize
+	}
+	if pageSize > maxPageSize {
+		pageSize = maxPageSize
+	}
+	offset := (page - 1) * pageSize
+
+	listParams := queries.ListWordsParams{
+		Limit:      pageSize,
+		Offset:     offset,
+		CategoryID: 0,
+		Keyword:    "",
+	}
+	countParams := queries.CountWordsParams{
+		CategoryID: 0,
+		Keyword:    "",
+	}
+	if req.CategoryID != nil && *req.CategoryID > 0 {
+		listParams.CategoryID = *req.CategoryID
+		countParams.CategoryID = *req.CategoryID
+	}
+	if req.Keyword != "" {
+		listParams.Keyword = req.Keyword
+		countParams.Keyword = req.Keyword
+	}
+
+	rows, err := s.q.ListWords(ctx, listParams)
+	if err != nil {
+		return models.ListWordsResponse{}, err
+	}
+	total, err := s.q.CountWords(ctx, countParams)
+	if err != nil {
+		return models.ListWordsResponse{}, err
+	}
+
+	words := make([]models.Word, len(rows))
+	for i, r := range rows {
+		words[i] = toWord(r)
+	}
+
+	totalPages := int32(math.Ceil(float64(total) / float64(pageSize)))
+	return models.ListWordsResponse{
+		Words:      words,
+		Total:      total,
+		Page:       page,
+		PageSize:   pageSize,
+		TotalPages: totalPages,
+	}, nil
+}
+
+// Get returns a single word by id.
+func (s *WordService) Get(ctx context.Context, id int32) (models.Word, error) {
+	row, err := s.q.GetWord(ctx, id)
+	if err != nil {
+		return models.Word{}, err
+	}
+	return toWord(row), nil
+}
+
+// Create creates a new word.
+func (s *WordService) Create(ctx context.Context, req models.CreateWordRequest) (models.Word, error) {
+	row, err := s.q.CreateWord(ctx, queries.CreateWordParams{
+		CategoryID:      req.CategoryID,
+		Word:            req.Word,
+		Translation:     req.Translation,
+		Phonetic:        req.Phonetic,
+		ExampleSentence: req.ExampleSentence,
+		AudioUrl:        req.AudioURL,
+		ImageUrl:        req.ImageURL,
+		SortOrder:       req.SortOrder,
+	})
+	if err != nil {
+		return models.Word{}, err
+	}
+	return toWord(row), nil
+}
+
+// Update updates an existing word.
+func (s *WordService) Update(ctx context.Context, id int32, req models.UpdateWordRequest) (models.Word, error) {
+	row, err := s.q.UpdateWord(ctx, queries.UpdateWordParams{
+		ID:              id,
+		CategoryID:      req.CategoryID,
+		Word:            req.Word,
+		Translation:     req.Translation,
+		Phonetic:        req.Phonetic,
+		ExampleSentence: req.ExampleSentence,
+		AudioUrl:        req.AudioURL,
+		ImageUrl:        req.ImageURL,
+		SortOrder:       req.SortOrder,
+	})
+	if err != nil {
+		return models.Word{}, err
+	}
+	return toWord(row), nil
+}
+
+// Delete removes a word by id.
+func (s *WordService) Delete(ctx context.Context, id int32) error {
+	return s.q.DeleteWord(ctx, id)
+}
+
+func toWord(w queries.Word) models.Word {
+	return models.Word{
+		ID:              w.ID,
+		CategoryID:      w.CategoryID,
+		Word:            w.Word,
+		Translation:     w.Translation,
+		Phonetic:        nullString(w.Phonetic),
+		ExampleSentence: nullString(w.ExampleSentence),
+		AudioURL:        nullString(w.AudioUrl),
+		ImageURL:        nullString(w.ImageUrl),
+		SortOrder:       w.SortOrder,
+		CreatedAt:       w.CreatedAt,
+		UpdatedAt:       w.UpdatedAt,
+	}
+}
+
+func nullString(s *string) *string {
+	if s == nil {
+		return nil
+	}
+	v := *s
+	return &v
+}