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.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/activity/service.go

@@ -3,6 +3,7 @@ package activity
 import (
 	"context"
 	"errors"
+	"time"
 
 	"bookra/apps/backend/internal/db"
 	"bookra/apps/backend/internal/domain"
@@ -35,6 +36,11 @@ func (s *Service) List(ctx context.Context, principal domain.Principal, limit in
 	if err != nil {
 		return domain.ActivityFeedResponse{}, err
 	}
+	readAt, err := s.repo.GetNotificationCursor(ctx, membership.Tenant.ID, principal.Subject)
+	if err != nil {
+		return domain.ActivityFeedResponse{}, err
+	}
+	unread := 0
 	events := make([]domain.ActivityEvent, 0, len(records))
 	for _, rec := range records {
 		event := domain.ActivityEvent{
@@ -49,7 +55,24 @@ func (s *Service) List(ctx context.Context, principal domain.Principal, limit in
 		if rec.EntityID != nil {
 			event.EntityID = *rec.EntityID
 		}
+		if readAt == nil || rec.CreatedAt.After(*readAt) {
+			unread++
+		}
 		events = append(events, event)
 	}
-	return domain.ActivityFeedResponse{Events: events}, nil
+	return domain.ActivityFeedResponse{Events: events, UnreadCount: unread, ReadAt: readAt}, nil
+}
+
+// MarkAllRead persists the user's cursor — events at or before now count as
+// read for this subject on every subsequent load.
+func (s *Service) MarkAllRead(ctx context.Context, principal domain.Principal) (time.Time, error) {
+	membership, err := s.repo.GetTenantMembershipBySubject(ctx, principal.Subject)
+	if err != nil {
+		return time.Time{}, ErrTenantMembership
+	}
+	readAt := time.Now().UTC()
+	if err := s.repo.SetNotificationCursor(ctx, membership.Tenant.ID, principal.Subject, readAt); err != nil {
+		return time.Time{}, err
+	}
+	return readAt, nil
 }