Forum 4 Direct messages

DMs introduce considerably more infrastructure:
```
conversations
↓
conversation_members
↓
messages
```
And then we need to think about:
- starting a conversation from a user's profile
- sending messages
- receiving messages
- unread messages
- message timestamps
- deleting messages
- blocking users
- potentially realtime subscriptions
- RLS policies in Supabase
- notification integration

So basicly this should perhaps be a project of its own instead being a sub issue to the forum project. But for now lets treat it as a "nice to have" feature and put last on the list of things to add for the forum

Start by reviewing the existing forum project and Supabase schema and RLS setup; the issue names no files, tests, or entry points. Clarify which conversation, membership, messaging, unread, deletion, blocking, realtime, and notification requirements are in scope before implementation, and define completion criteria for the selected scope.

Written by the indexing model from the issue text.