feat: add redis_password support

[carlowahlstedt]

Summary

Adds optional redis_password support to RedisStorage so bots can connect to password-protected Redis instances.

Changes

• signalbot/storage.py: RedisStorage.__init__ now accepts an optional password: str | None = None parameter, which is forwarded to redis.Redis().
• signalbot/bot.py: Bot initialization reads redis_password from the storage config (via .get(), so it remains optional) and passes it to RedisStorage.
• README.md: Documents the new redis_password config option under the Persistent storage section.

Usage

bot = SignalBot({
    "signal_service": "127.0.0.1:8080",
    "phone_number": "+49123456789",
    "storage": {
        "redis_host": "redis",
        "redis_port": 6379,
        "redis_password": "your_password",
    },
})

The field is optional — existing configs without redis_password continue to work unchanged (password defaults to None).

🤖 Generated with Claude Code

[carlowahlstedt]

Closing in favor of a conflict-free PR based on upstream/main.