feat: csrf Redis storage adapter

[FumingPower3925]

Summary

Add a Redis-backed csrf.Storage implementation for server-side CSRF tokens.

Context

The CSRF middleware defines:

type Storage interface {
    Get(key string) (string, bool)
    Set(key string, token string, expiry time.Duration)
    Delete(key string)
}

Currently uses an in-memory sharded map. For multi-instance deployments, server-side CSRF tokens must be shared.

Scope

• Implement RedisStorage satisfying csrf.Storage
• Use Redis GET/SETEX/DEL with key prefix (default: csrf:)
• Use celeris's native Redis driver