feat(k6): Stress test messaging

[Harxhit]

Goal

Stress the messaging tier and confirm it degrades gracefully, then run the
weekly review.

Tasks

• Load test chat: many senders across ≥2 pods; confirm fanout latency stays acceptable
• Confirm outbound emit batching engages under load (no per-message storm)
• Burst reactions; confirm aggregation + 10s rollups hold (no write storm)
• Verify rate limits reject abusive clients without harming normal ones
• Verify presence stays accurate under churn (mass join/leave)
• Confirm backpressure behavior (coalesce/drop) under extreme load per §8
• Fill in review.md and move unfinished work to its Backlog

Acceptance criteria

• Chat/reaction/presence remain correct and bounded under load
• Rate limiting and moderation hold under abuse
• Week 5 review completed; backlog captured

Then

Open the Week 5 review.