Friend/block model decision for social layer

## Summary
Decide the friend/block model for RFChat's social layer before broader DM and profile work expands.

## Scope
- Decide whether RFChat should support friend relationships, block relationships, both, or a deliberately narrower model.
- Define how blocking affects DMs, group DMs, mentions, profile visibility, and future discovery surfaces.
- Define how much of Discord's social layer is worth reproducing in a single-guild-per-deployment world.
- Produce a concrete product decision that downstream issues can build on.

## Acceptance Criteria
- [ ] The product decision is documented in enough detail to drive later implementation issues.
- [ ] The chosen model defines user-visible behavior for blocking across the main interaction surfaces.
- [ ] Non-goals are explicit so later issues do not silently assume full Discord social parity.
- [ ] Follow-up implementation issues can reference this decision without reopening core scope questions.

## Operational Notes
- Single-guild deployments may not need all of Discord's social graph complexity.
- Any decision here should prioritize predictable privacy behavior over novelty.