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/migrations/000001_init_schema.up.sql

@@ -0,0 +1,25 @@
+CREATE TABLE IF NOT EXISTS categories (
+    id          SERIAL PRIMARY KEY,
+    name        VARCHAR(100) NOT NULL,
+    sort_order  INT NOT NULL DEFAULT 0,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    UNIQUE (name)
+);
+
+CREATE TABLE IF NOT EXISTS words (
+    id               SERIAL PRIMARY KEY,
+    category_id      INT NOT NULL REFERENCES categories(id) ON DELETE CASCADE,
+    word             VARCHAR(255) NOT NULL,
+    translation      VARCHAR(255) NOT NULL,
+    phonetic         VARCHAR(255),
+    example_sentence TEXT,
+    audio_url        VARCHAR(500),
+    image_url        VARCHAR(500),
+    sort_order       INT NOT NULL DEFAULT 0,
+    created_at       TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at       TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_words_category_id ON words(category_id);
+CREATE INDEX IF NOT EXISTS idx_words_word ON words(word);