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>

scripts/setup-pb-collections.mjs

@@ -56,7 +56,9 @@ const COLLECTIONS = [
     ],
   },
   {
-    name: 'quiz_banks',
+    // Question bank: one record per topic with a `questions` JSON array.
+    // Shared across all users; admin-curated; powers on-demand topic tests.
+    name: 'question_bank',
     type: 'base',
     ...OPEN_RULES,
     fields: [
@@ -65,6 +67,30 @@ const COLLECTIONS = [
       ...AUTODATE_FIELDS,
     ],
   },
+  {
+    // On-demand attempts log: one record per completed topic test.
+    // Powers the weekly-cap aggregation and the Contributions feed.
+    name: 'on_demand_attempts',
+    type: 'base',
+    ...OPEN_RULES,
+    fields: [
+      { name: 'user_id',        type: 'text',   required: true },
+      { name: 'topic_id',       type: 'text',   required: true },
+      { name: 'topic_label',    type: 'text',   required: false },
+      { name: 'question_ids',   type: 'json',   required: false },
+      { name: 'score',          type: 'number', required: false },
+      { name: 'total',          type: 'number', required: false },
+      { name: 'percentage',     type: 'number', required: false },
+      { name: 'time_used',      type: 'number', required: false },
+      { name: 'unseen_correct', type: 'number', required: false },
+      { name: 'points_earned',  type: 'number', required: false },
+      { name: 'capped',         type: 'bool',   required: false },
+      { name: 'iso_week',       type: 'text',   required: false },
+      { name: 'completed_at',   type: 'text',   required: false },
+      { name: 'breakdown',      type: 'json',   required: false },
+      ...AUTODATE_FIELDS,
+    ],
+  },
   {
     name: 'sources',
     type: 'base',