User reports and moderation queue

## Summary
Implement user reports and a moderation queue for handling harmful content and member complaints.

## Scope
- Allow users to report messages and members from the product UI.
- Provide a moderator queue for triage, assignment/status, and resolution.
- Link reports to underlying messages, users, and moderation actions.
- Preserve enough evidence to review a report even if surrounding context changes.

## Acceptance Criteria
- [ ] Users can submit reports with clear reason and target context.
- [ ] Moderators can review open reports from a dedicated queue.
- [ ] Reports can be resolved through direct moderation actions or explicit dismissal.
- [ ] The queue preserves enough history to understand what happened later.

## Operational Notes
- Self-hosted communities need a lightweight but real moderation workflow, not just ad hoc moderator memory.
- Report retention and privacy boundaries should be explicit.