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

apps/backend/internal/domain/models.go

@@ -553,7 +553,9 @@ type ActivityEvent struct {
 }
 
 type ActivityFeedResponse struct {
-	Events []ActivityEvent `json:"events"`
+	Events      []ActivityEvent `json:"events"`
+	UnreadCount int             `json:"unreadCount"`
+	ReadAt      *time.Time      `json:"readAt,omitempty"`
 }
 
 // StaffMetric aggregates appointment workload for one staff member over a