feat(quiz): variety in quiz question selection

Expand the seeded pool from 12 to 36 questions (migration 0020) and
select 10 per session preferring questions the couple has never seen,
then least recently used, with random tiebreaks, so repeat quizzes stop
serving the same set.

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

docs/quiz.md

@@ -9,7 +9,7 @@ once both have finished.
 
 | Concern | Location |
 |---|---|
-| Schema and seed content | `apps/api/drizzle/0019_quiz.sql` |
+| Schema and seed content | `apps/api/drizzle/0019_quiz.sql`, `apps/api/drizzle/0020_quiz_question_pool.sql` |
 | HTTP contract | `apps/api/src/domains/quiz/routes.ts` |
 | Session and scoring rules | `apps/api/src/domains/quiz/repository.ts` |
 | Pure scoring function | `apps/api/src/domains/quiz/service.ts` (`computeQuizScore`) |
@@ -30,6 +30,15 @@ quiz while one is already active returns that same session (`200`) instead of
 creating a second one; two simultaneous starts race safely because the
 partial unique index, not application locking, is the source of truth.
 
+When a new session is created, the 10 questions are chosen from the active
+pool with a couple-scoped preference order: questions this couple has never
+had in any prior session (completed or active) come first, then the
+least-recently-used ones, with a random tiebreak within each group. This
+keeps repeat quizzes varied instead of always drawing the same
+`sort_order`-first questions. The seeded pool (`0019_quiz.sql` plus
+`0020_quiz_question_pool.sql`) has 36 active questions, so two consecutive
+sessions for the same couple are guaranteed not to overlap.
+
 Answers are submitted as one complete batch per call (`quiz_answers`, upsert
 on `(session_id, user_id, question_id)`). A submission must cover every
 question in the session exactly once, with every index inside that
@@ -73,8 +82,9 @@ completed session.
 - `apps/api/test/quiz.routes.test.ts`: HTTP contract and error mapping
 - `apps/api/test/quiz.integration.test.ts`: real PostgreSQL, covering the full
   happy path with a hand-computed score, idempotent session start, couple
-  isolation, incomplete answer set rejection, and result blocked before
-  completion
+  isolation, incomplete answer set rejection, result blocked before
+  completion, no question overlap between a completed session and the next
+  one for the same couple, and that selection is scoped per couple
 
 ## Mobile client