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/sql/category.sql

@@ -0,0 +1,25 @@
+-- name: ListCategories :many
+SELECT id, name, sort_order, created_at, updated_at
+FROM categories
+ORDER BY sort_order ASC, id ASC;
+
+-- name: GetCategory :one
+SELECT id, name, sort_order, created_at, updated_at
+FROM categories
+WHERE id = $1;
+
+-- name: CreateCategory :one
+INSERT INTO categories (name, sort_order)
+VALUES ($1, $2)
+RETURNING id, name, sort_order, created_at, updated_at;
+
+-- name: UpdateCategory :one
+UPDATE categories
+SET name = $2,
+    sort_order = $3,
+    updated_at = NOW()
+WHERE id = $1
+RETURNING id, name, sort_order, created_at, updated_at;
+
+-- name: DeleteCategory :exec
+DELETE FROM categories WHERE id = $1;