Implement the proposal discussion module

## Outcome

Users can participate in safe, accessible proposal discussions based on the approved design.

## Scope

- Implement the approved discussion lifecycle and permissions
- Implement moderation, abuse handling, auditability, and notifications
- Integrate the module with proposal context and profiles
- Add security, accessibility, integration, and operational tests

## Acceptance criteria

- [ ] Scope is decomposed into independently deliverable child issues.
- [ ] Architecture and API decisions are linked from this issue.
- [ ] Test, documentation, observability, rollout, and failure behavior are defined where applicable.
- [ ] Epic exit criteria are verified before closure.

## Planning note

This is a 2026 GovTool revamp work item. Its child features and tasks should be added as the scope is refined.

Start by decomposing the proposal discussion lifecycle, permissions, moderation, abuse handling, auditability, notifications, proposal context, and profile integration into independently deliverable child issues. Link the architecture and API decisions, then define applicable tests, documentation, observability, rollout, and failure behavior before verifying the epic exit criteria.

Written by the indexing model from the issue text.