feat(notifications): persistent read cursor + delivery history

notification_cursors table stores one read-cursor per user per tenant;
GET /v1/activity returns unreadCount + readAt, POST /v1/activity/read
persists the cursor. Reminder dispatches now append reminder.sent /
reminder.failed system events so delivery history reaches the feed.

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project.md

@@ -188,6 +188,13 @@
   - demo repository seeds four trailing-window bookings (3 confirmed + 1 cancelled) via `NewRepository`'s demo branch only — unit tests constructing `NewMemoryRepository` directly are unaffected
   - verified live: endpoint returns 4/3/1, 75% utilization, 3 600 Kč; `days=7` and `days=999` normalize correctly; browser-checked both locales and mobile wrap, console clean; focused aggregation test passes
   - remaining: live staging validation on Neon + pgx repository path
+- `[done/partial]` Milestone 27: Persistent notification center
+  - `notification_cursors` table (migration 00017) — one read-cursor per (tenant, user_subject); per-event flags rejected deliberately, the feed is an audit list not a per-item inbox
+  - `GET /v1/activity` now returns `unreadCount` + `readAt` alongside events; `POST /v1/activity/read` persists the cursor and returns it — mark-all-read survives reloads and sessions
+  - delivery history joins the feed: `DispatchDue` appends `reminder.sent` / `reminder.failed` system events (entity = booking, summary = `{customer} — {channel}`) after each delivery-log write, best-effort so notifications never fail on audit
+  - dropdown derives per-item read state from `createdAt <= readAt` — only post-cursor events carry the unread dot; per-item clicks still mark locally for the session; the billing/trial nag stays session-scoped (state badge, not an event)
+  - verified live: 3 unread → mark → 0 → new booking → exactly 1 unread; browser-confirmed the badge and dimming persist across reload; focused tests cover cursor lifecycle and the reminder event
+  - remaining: live staging validation on Neon + pgx repository path
 
 ## Overview
 Bookra is a Czech-first booking SaaS for small local service businesses. It targets salons, massage therapists, small clinics, repair shops, and small studios/clubs that need a simple public booking flow, internal schedule management, reminders, and basic business reporting without becoming a full ERP.
@@ -440,7 +447,7 @@ Shortlist of worthwhile directions once v1 stabilizes — kept deliberately smal
 - **Google Calendar sync**: M22 shipped read-only ICS feeds; two-way sync (push bookings into Google, pull busy blocks back as availability exceptions) needs OAuth and conflict rules — larger commitment, defer until a concrete request.
 - **Recurring class schedules**: templates currently schedule sessions one at a time; a weekly recurrence rule with per-session overrides would cut setup time for studios.
 - **Booking notes intake**: structured intake fields per service (e.g. "first visit?", "injuries?") instead of the single free-text notes field — still schema-light.
-- **Persistent notification center**: M24 put real activity events in the dropdown, but read state is still in-memory and there's no delivery history; a `notifications` table (or read-cursor on `activity_events`) would persist both.
+- **Browser push notifications**: M27 persists read state and the feed covers deliveries; Web Push (service worker + VAPID) would notify owners about new bookings without keeping the dashboard open — defer until a concrete request, email already covers off-site reach.
 - **Capacity-based utilization**: M26 measures utilization as the confirmed share of bookings; a truer measure compares booked minutes against each member's working-hours rules — needs availability-rule integration in the metrics query.
 - **Report-style exports**: M25 shipped raw row exports; a summarized report (per-staff metrics, revenue by service, period comparisons) can reuse `GetStaffMetrics`-style aggregation once the metric set stabilizes.