feat(quiz): couples quiz with shareable watermarked results card

Replaces the notify-me quiz stub with a full loop: quiz sessions per
couple, answer-and-guess flow, dual-direction scoring, and a score-only
share card (no question or answer content) captured via ViewShot with
Sauci branding.

API: migration 0019 (quiz_questions, quiz_sessions, quiz_answers, 12
seeded questions), new quiz domain with session start / current /
answers / result routes, couple isolation, route + integration +
scoring unit tests.

Mobile: src/features/quiz feature (intro, per-question self+guess flow,
waiting-for-partner nudge, results with breakdown), quizStore with
sign-out clearing, QuizShareModal following SharePreviewModal, discovery
QuizTile, analytics events, and tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/index.md

@@ -24,6 +24,7 @@ historical notes. Agents should load only the section relevant to their task.
 - [Redemption codes](product/redemption.md)
 - [Mobile release process](releasing.md)
 - [Dares loop](dares-loop.md)
+- [Couples quiz](quiz.md)
 
 ## Feature and implementation references