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/repository/queries/querier.go

@@ -0,0 +1,25 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+
+package queries
+
+import (
+	"context"
+)
+
+type Querier interface {
+	CountWords(ctx context.Context, arg CountWordsParams) (int64, error)
+	CreateCategory(ctx context.Context, arg CreateCategoryParams) (Category, error)
+	CreateWord(ctx context.Context, arg CreateWordParams) (Word, error)
+	DeleteCategory(ctx context.Context, id int32) error
+	DeleteWord(ctx context.Context, id int32) error
+	GetCategory(ctx context.Context, id int32) (Category, error)
+	GetWord(ctx context.Context, id int32) (Word, error)
+	ListCategories(ctx context.Context) ([]Category, error)
+	ListWords(ctx context.Context, arg ListWordsParams) ([]Word, error)
+	UpdateCategory(ctx context.Context, arg UpdateCategoryParams) (Category, error)
+	UpdateWord(ctx context.Context, arg UpdateWordParams) (Word, error)
+}
+
+var _ Querier = (*Queries)(nil)