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/docker-compose.yml

@@ -0,0 +1,39 @@
+services:
+  db:
+    image: postgres:18.3-alpine3.23
+    container_name: language-db
+    environment:
+      POSTGRES_USER: language
+      POSTGRES_PASSWORD: language123
+      POSTGRES_DB: language
+    ports:
+      - "5432:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U language -d language"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  api:
+    build:
+      context: .
+      dockerfile: docker/Dockerfile
+      target: dev
+    container_name: language-api
+    environment:
+      APP_ENV: development
+      DATABASE_URL: postgres://language:language123@db:5432/language?sslmode=disable
+      HTTP_ADDR: :8080
+    ports:
+      - "8080:8080"
+    volumes:
+      - .:/app
+    depends_on:
+      db:
+        condition: service_healthy
+    command: ["air", "-c", ".air.toml"]
+
+volumes:
+  postgres_data: