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;

backend/sql/word.sql

@@ -0,0 +1,50 @@
+-- name: ListWords :many
+SELECT w.id, w.category_id, w.word, w.translation, w.phonetic,
+       w.example_sentence, w.audio_url, w.image_url, w.sort_order,
+       w.created_at, w.updated_at
+FROM words w
+WHERE
+    (@category_id::int = 0 OR w.category_id = @category_id)
+    AND (@keyword::text = '' OR w.word ILIKE '%' || @keyword || '%' OR w.translation ILIKE '%' || @keyword || '%')
+ORDER BY w.sort_order ASC, w.id ASC
+LIMIT $1 OFFSET $2;
+
+-- name: CountWords :one
+SELECT COUNT(*) AS total
+FROM words w
+WHERE
+    (@category_id::int = 0 OR w.category_id = @category_id)
+    AND (@keyword::text = '' OR w.word ILIKE '%' || @keyword || '%' OR w.translation ILIKE '%' || @keyword || '%');
+
+-- name: GetWord :one
+SELECT id, category_id, word, translation, phonetic,
+       example_sentence, audio_url, image_url, sort_order,
+       created_at, updated_at
+FROM words
+WHERE id = $1;
+
+-- name: CreateWord :one
+INSERT INTO words (category_id, word, translation, phonetic, example_sentence, audio_url, image_url, sort_order)
+VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+RETURNING id, category_id, word, translation, phonetic,
+          example_sentence, audio_url, image_url, sort_order,
+          created_at, updated_at;
+
+-- name: UpdateWord :one
+UPDATE words
+SET category_id = $2,
+    word = $3,
+    translation = $4,
+    phonetic = $5,
+    example_sentence = $6,
+    audio_url = $7,
+    image_url = $8,
+    sort_order = $9,
+    updated_at = NOW()
+WHERE id = $1
+RETURNING id, category_id, word, translation, phonetic,
+          example_sentence, audio_url, image_url, sort_order,
+          created_at, updated_at;
+
+-- name: DeleteWord :exec
+DELETE FROM words WHERE id = $1;