fix: 添加缺失的PASSWORD环境变量以修复500错误

- 在 wrangler.toml 中为生产和预览环境添加 PASSWORD 变量
- PASSWORD 是中间件验证的必需变量，缺失会导致500错误
- 更新部署指南，强调 PASSWORD 必须设置为 Plain text 类型
- 添加详细的环境变量类型设置说明和故障排除步骤
- PASSWORD 用于用户认证和签名验证，是系统核心安全机制

wrangler.toml

@@ -20,6 +20,7 @@ NEXT_PUBLIC_SITE_NAME = "KatelyaTV"
 NEXT_PUBLIC_SITE_DESCRIPTION = "高性能影视播放平台"
 NEXTAUTH_URL = "https://your-domain.pages.dev"
 USERNAME = "katelya"
+PASSWORD = "your-secure-password-here"
 IMAGE_PROXY_ENABLED = "true"
 CACHE_TTL = "3600"
 CORS_ORIGIN = "*"
@@ -47,5 +48,6 @@ NEXT_PUBLIC_STORAGE_TYPE = "d1"
 NEXT_PUBLIC_SITE_NAME = "KatelyaTV"
 NEXT_PUBLIC_SITE_DESCRIPTION = "高性能影视播放平台"
 USERNAME = "katelya"
+PASSWORD = "your-secure-password-here"
 IMAGE_PROXY_ENABLED = "true"
 NODE_ENV = "production"