Slash commands, bots, and webhooks

## Summary
Implement server-local bots, slash commands, and webhooks with Discord-like behavior where it makes sense for a single-guild deployment.

## Scope
- Support inbound webhooks, bot identities, and slash-command style interactions.
- Define permissions, visibility, and execution semantics for integrations inside one guild.
- Provide admin UX for installing/configuring local integrations without a global app marketplace.
- Keep the interaction model predictable for end users and server operators.

## Acceptance Criteria
- [ ] The product supports a coherent integration model for webhooks, bots, and slash-command interactions.
- [ ] Permissions and visibility for integrations are enforced consistently.
- [ ] Server admins can manage integrations from product UI or supported admin APIs.
- [ ] End users can discover and use slash-command style interactions without learning a separate mental model.

## Operational Notes
- RFChat should optimize for self-hosted server-local integrations first, not Discord's global application platform.
- Lower-level streaming or transport details should live in dedicated follow-up issues.

The issue names no files, tests, or entry points. Start by mapping the existing server integration and admin-UI surfaces, then clarify the interaction, permission, visibility, and configuration design before implementation. Done means coherent server-local webhooks, bots, and slash-command interactions that admins can manage and end users can discover.

Written by the indexing model from the issue text.