Phase 2: learning engine — history, SRS review, mistakes, stats, exam, reminders

Answer history and schedules
- Every answer is recorded (deck, activity, topic, correct/wrong, time)
  in a versioned IndexedDB log (freezmo-learning v1, stores answers +
  srs), mirrored in memory for synchronous React reads and capped at
  20k answers (oldest pruned). StudyFeed records as it grades.
- Light SM-2 schedule per activity: 1 day, 3 days, then interval x
  ease; wrong answers return in ten minutes with the ease nudged down.

Review, mistakes, stats
- Library 'Smart review' card: due count, mistakes count (latest
  answer wrong — clears itself when answered correctly), today's goal
  progress. Review sessions freeze their item list at start.
- New /review (due across decks, ?mistakes=1 global or ?deck= per
  deck) built on a shared SessionFeed with the study feed's snap/keyboard
  mechanics, wake lock and a results slide.
- New /stats: day streak, five-week calendar, accuracy per subject and
  per deck, weakest topics (>=3 answers), daily-goal progress.
- Daily goal configurable in Settings (10/20/30/50).

Exam mode
- New /exam: pick decks, question count, optional timer; no hints
  (a NoHints context hides hint rows and the self-check escape);
  results with score, per-deck and per-topic breakdown; wrong answers
  land in the mistakes set through the normal history.

Reminders (conditional)
- VAPID-gated: GET/POST/DELETE /api/push and POST /api/push/test with
  the standard route pattern, RATE_LIMIT_PUSH, zod, auth before body.
  Subscriptions stored server-side as endpoint + keys only (PUSH_DIR).
  UI hidden entirely without keys; iOS install note; service worker
  push/notificationclick handlers; scripts/send-reminders.mjs for a
  cron/systemd timer; web-push added as the one server dependency.

Verified: new phase-2 suite 42/42 (history/SRS shapes from a real study
run, mistakes clearing, review session, stats math, exam timer and
breakdowns, VAPID-less hidden state plus the configured
subscribe/validate/test/delete path against a second instance) plus
regression 74/74, mobile 52/52, wizard 80/80, declutter 116/116,
animation 8/8, native 35/35, phase1 38/38, i18n 25/25. Key parity
709/709. Lint + build clean. SW freezmo-v26.

.env.example

@@ -45,6 +45,21 @@ TEXT_MODEL=gpt-oss:120b-cloud
 # RATE_LIMIT_HEALTH=90
 # Encrypted sync (no model calls, but shouldn't be hammered).
 # RATE_LIMIT_SYNC=60
+# Push subscription changes and test sends.
+# RATE_LIMIT_PUSH=30
+
+# PUSH REMINDERS (optional) ----------------------------------------------------
+# Web Push reminders only appear in the UI when BOTH VAPID keys are set.
+# Generate them once with:  npx web-push generate-vapid-keys
+# VAPID_PUBLIC_KEY=
+# VAPID_PRIVATE_KEY=
+# VAPID_SUBJECT=mailto:you@example.com
+# Where subscriptions are stored (endpoint + keys only, no personal data).
+# PUSH_DIR=.freezmo/push
+# The daily sender is a script you schedule yourself (cron/systemd timer):
+#   node scripts/send-reminders.mjs
+# PUSH_REMINDER_TITLE=Time to review
+# PUSH_REMINDER_BODY=Your decks are waiting — a few due cards keep the streak alive.
 
 # ENCRYPTED SYNC STORAGE -------------------------------------------------------
 # Opt-in sync uploads an opaque AES-GCM blob; the server can't read it and