Spam controls and message rate limiting

## Summary
Implement spam controls and message rate limiting with Discord-like user feedback and moderator controls.

## Scope
- Enforce server-side message rate limits for repeated sends.
- Support channel-level slowmode and other user-visible throttles.
- Detect and block obvious burst spam patterns such as duplicate sends or excessive mentions.
- Surface actionable feedback to users and moderators when limits are hit.

## Acceptance Criteria
- [ ] Rate limits are enforced in realtime and API message-send paths.
- [ ] Slowmode is configurable per channel and visible to affected users.
- [ ] Users see clear retry timing or explanation when blocked.
- [ ] Moderators/admins can configure and inspect the relevant controls without digging into logs.

## Operational Notes
- Counters should be backed by infrastructure appropriate for multi-process and eventually multi-node deployments.
- Abuse controls must avoid turning transient reconnect bursts into false positives.