Add Redis-based one-time download token support

Integrate Redis as a backend for one-time download token validation, with in-memory fallback. Added RedisManager for connection lifecycle, TokenStore for atomic token usage checks, and related configuration via environment variables. Updated download flow to ensure tokens are single-use, and improved API robustness for batch operations. Updated dependencies to include redis and cachetools.

service/env.md

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+# 环境变量字段
+
+- `MODE` str 运行模式，默认 `master`
+  - `master` 主机模式
+  - `slave` 从机模式
+- `DEBUG` bool 是否开启调试模式，默认 `false`
+- `DATABASE_URL`: 数据库连接信息，默认 `sqlite+aiosqlite:///disknext.db`
+- `REDIS_HOST`: Redis 主机地址
+- `REDIS_PORT`: Redis 端口
+- `REDIS_PASSWORD`: Redis 密码
+- `REDIS_DB`: Redis 数据库
+- `REDIS_PROTOCOL`