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/api/src/domains/quiz/service.ts

@@ -0,0 +1,41 @@
+/**
+ * One partner's guess of the other's self-answer, for a single question. Scoring
+ * always runs both directions: A guessing B and B guessing A each count separately,
+ * so a couple who only ever gets half the guesses right lands at 50%, not 100%.
+ */
+export interface QuizAnswerPair {
+  questionId: string;
+  aSelfIndex: number;
+  aGuessIndex: number;
+  bSelfIndex: number;
+  bGuessIndex: number;
+}
+
+export interface QuizScoreQuestionResult extends QuizAnswerPair {
+  aGuessedRight: boolean;
+  bGuessedRight: boolean;
+}
+
+export interface QuizScoreResult {
+  scorePercent: number;
+  perQuestion: QuizScoreQuestionResult[];
+}
+
+/**
+ * Pure scoring function for a completed quiz session. Each question contributes
+ * up to two hits (partner A's guess about B, and B's guess about A), so the
+ * denominator is twice the question count.
+ */
+export function computeQuizScore(pairs: QuizAnswerPair[]): QuizScoreResult {
+  const perQuestion = pairs.map((pair) => ({
+    ...pair,
+    aGuessedRight: pair.aGuessIndex === pair.bSelfIndex,
+    bGuessedRight: pair.bGuessIndex === pair.aSelfIndex,
+  }));
+  const hits = perQuestion.reduce(
+    (sum, question) => sum + (question.aGuessedRight ? 1 : 0) + (question.bGuessedRight ? 1 : 0),
+    0,
+  );
+  const scorePercent = pairs.length === 0 ? 0 : Math.round((100 * hits) / (2 * pairs.length));
+  return { scorePercent, perQuestion };
+}