docs: design doc for fup og fakta game engine + platform architecture

Captures all brainstormed decisions:
- Pluggable game cartridge platform (GameDriver interface)
- Celery + Redis timer-driven phase transitions
- Session owner play/pause/exit controls (no skip)
- Escalating scoring per round, incremental reveal scoring
- Emoji reactions during guess phase → post-game awards
- Relational per-user config presets with game-specific models
- Ephemeral game state (no persistence after exit/finish)
- Full WebSocket event reference and data lifecycle

Also: updated TODO.md (WebSocket done, persisted answers done),
created CLAUDE.md, and PROMPT.md for ralph-loop.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/settings.json

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