feat: on-demand topic tests with shared question bank

Adds a /topic-test route where learners can take a 5-question test on any
eligible topic at any time. Questions come from a shared, admin-curated
question_bank — no LLM calls on the user path. Points feed the existing
leaderboard with a 10pt/topic/week cap (per ISO week) so the bank can't be
farmed, and repeats from a thin bank yield 0 points.

- New PB collections: question_bank, on_demand_attempts (+ migrations and
  setup-pb-collections.mjs entries).
- db.js: un-deprecates getQuizBank/setQuizBank against question_bank so the
  existing admin TestManager panel becomes functional again; adds
  saveOnDemandAttempt, getOnDemandPointsThisWeek, getUserSeenQuestionIds,
  getAllOnDemandAttempts, isoWeekKey.
- testService.js: getEligibleTopicsForOnDemand, startOnDemandTest (unseen-
  first draw), finishOnDemandTest (cap enforcement + leaderboard upsert).
- New TopicTest page, route, and nav entry; weekly test flow untouched.
- Leaderboard folds on-demand 100%s into perfect-score count and merges
  on-demand attempts into the recent activity feed.
- Spec: docs/on-demand-tests-spec.md with ADRs and extension points.
- Tests: 5 new vitest cases covering eligibility, draw fallback, scoring,
  partial cap, and exhausted cap (85/85 total).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/db.js

@@ -105,14 +105,105 @@ export async function deleteContent(topicId) {
   } catch { /* no record, nothing to do */ }
 }
 
-// ── Quiz Banks (DEPRECATED — collection dropped) ────────────────────────────
+// ── Question Bank (shared, admin-curated, used by on-demand tests) ──────────
+// One PB record per topic, with `questions: json[]`. The legacy aliases
+// (getQuizBank etc.) are kept so existing admin UI code keeps working.
 
-/** @deprecated quiz_banks collection has been dropped. */
-export async function getQuizBank() { return []; }
-/** @deprecated quiz_banks collection has been dropped. */
-export async function setQuizBank() { return null; }
-/** @deprecated quiz_banks collection has been dropped. */
-export async function deleteQuestionFromBank() { return null; }
+export async function getQuestionBank(topicId) {
+  try {
+    const r = await pb.collection('question_bank').getFirstListItem(`topic_id="${topicId}"`);
+    return Array.isArray(r.questions) ? r.questions : [];
+  } catch { return []; }
+}
+
+export async function setQuestionBank(topicId, questions) {
+  return pbUpsert(
+    'question_bank',
+    `topic_id="${topicId}"`,
+    { questions },
+    { topic_id: topicId, questions },
+  );
+}
+
+export async function deleteQuestionFromBank(topicId, questionId) {
+  try {
+    const r = await pb.collection('question_bank').getFirstListItem(`topic_id="${topicId}"`);
+    const next = (r.questions || []).filter(q => q.id !== questionId);
+    return pb.collection('question_bank').update(r.id, { questions: next });
+  } catch { return null; }
+}
+
+// Legacy aliases — the admin TestManager UI and forceGenerateTopicQuestions
+// call these. Routing them at the new collection un-breaks the panel.
+export const getQuizBank = getQuestionBank;
+export const setQuizBank = setQuestionBank;
+
+// ── On-demand attempts ──────────────────────────────────────────────────────
+
+function isoWeekKey(date = new Date()) {
+  // Returns "YYYY-Www" for the ISO week containing `date`. Used for the
+  // per-(user, topic) weekly point cap aggregation.
+  const d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
+  const day = d.getUTCDay() || 7;
+  d.setUTCDate(d.getUTCDate() + 4 - day);
+  const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
+  const weekNo = Math.ceil((((d - yearStart) / 86400000) + 1) / 7);
+  return `${d.getUTCFullYear()}-W${String(weekNo).padStart(2, '0')}`;
+}
+
+export { isoWeekKey };
+
+export async function saveOnDemandAttempt(attempt) {
+  return pb.collection('on_demand_attempts').create({
+    user_id:        attempt.userId,
+    topic_id:       attempt.topicId,
+    topic_label:    attempt.topicLabel || '',
+    question_ids:   attempt.questionIds || [],
+    score:          attempt.score,
+    total:          attempt.total,
+    percentage:     attempt.percentage,
+    time_used:      attempt.timeUsed,
+    unseen_correct: attempt.unseenCorrect,
+    points_earned:  attempt.pointsEarned,
+    capped:         !!attempt.capped,
+    iso_week:       attempt.isoWeek,
+    completed_at:   attempt.completedAt,
+    breakdown:      attempt.breakdown || [],
+  });
+}
+
+export async function getOnDemandPointsThisWeek(userId, topicId) {
+  try {
+    const week = isoWeekKey();
+    const rows = await pb.collection('on_demand_attempts').getFullList({
+      filter: `user_id="${userId}" && topic_id="${topicId}" && iso_week="${week}"`,
+    });
+    return rows.reduce((sum, r) => sum + (r.points_earned || 0), 0);
+  } catch { return 0; }
+}
+
+export async function getUserSeenQuestionIds(userId, topicId) {
+  // Returns the Set of question_bank IDs this user has already been served
+  // for `topicId`. Used by the draw routine to prefer unseen questions and
+  // by finishOnDemandTest to zero out points for repeats.
+  try {
+    const rows = await pb.collection('on_demand_attempts').getFullList({
+      filter: `user_id="${userId}" && topic_id="${topicId}"`,
+      fields: 'question_ids',
+    });
+    const seen = new Set();
+    for (const r of rows) {
+      for (const id of (r.question_ids || [])) seen.add(id);
+    }
+    return seen;
+  } catch { return new Set(); }
+}
+
+export async function getAllOnDemandAttempts() {
+  try {
+    return await pb.collection('on_demand_attempts').getFullList({ sort: '-created' });
+  } catch { return []; }
+}
 
 // ── Sources ──────────────────────────────────────────────────────────────────