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>

apps/mobile/app/(app)/index.tsx

@@ -8,7 +8,7 @@ import * as Haptics from 'expo-haptics';
 import { useAuthStore, useMatchStore, usePacksStore, useSubscriptionStore } from '../../src/store';
 import { GradientBackground } from '../../src/components/ui';
 import { Paywall } from '../../src/components/paywall';
-import { CompactHeader, ContentRow, DaresTile, LiveDrawTile, PendingQuestionsCard } from '../../src/components/discovery';
+import { CompactHeader, ContentRow, DaresTile, LiveDrawTile, PendingQuestionsCard, QuizTile } from '../../src/components/discovery';
 import { StreakDisplay } from '../../src/components/StreakDisplay';
 import { Events } from '../../src/lib/analytics';
 import { colors, spacing, typography } from '../../src/theme';
@@ -166,6 +166,9 @@ export default function DiscoveryScreen() {
           {/* Dares */}
           <DaresTile delay={200} />
 
+          {/* Quiz */}
+          <QuizTile delay={225} />
+
           {/* Content Rows by Category */}
           {packsByCategory.map((group, index) => (
             <ContentRow