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/models.go

@@ -0,0 +1,31 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+
+package queries
+
+import (
+	"time"
+)
+
+type Category struct {
+	ID        int32     `json:"id"`
+	Name      string    `json:"name"`
+	SortOrder int32     `json:"sort_order"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+type Word struct {
+	ID              int32     `json:"id"`
+	CategoryID      int32     `json:"category_id"`
+	Word            string    `json:"word"`
+	Translation     string    `json:"translation"`
+	Phonetic        *string   `json:"phonetic"`
+	ExampleSentence *string   `json:"example_sentence"`
+	AudioUrl        *string   `json:"audio_url"`
+	ImageUrl        *string   `json:"image_url"`
+	SortOrder       int32     `json:"sort_order"`
+	CreatedAt       time.Time `json:"created_at"`
+	UpdatedAt       time.Time `json:"updated_at"`
+}