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/config/config.go

@@ -0,0 +1,23 @@
+package config
+
+import (
+	"fmt"
+
+	"github.com/kelseyhightower/envconfig"
+)
+
+// Config holds all application configuration loaded from environment variables.
+type Config struct {
+	AppEnv      string `envconfig:"APP_ENV" default:"development"`
+	HTTPAddr    string `envconfig:"HTTP_ADDR" default:":8080"`
+	DatabaseURL string `envconfig:"DATABASE_URL" required:"true"`
+}
+
+// Load parses environment variables into Config.
+func Load() (*Config, error) {
+	var cfg Config
+	if err := envconfig.Process("", &cfg); err != nil {
+		return nil, fmt.Errorf("failed to load config: %w", err)
+	}
+	return &cfg, nil
+}