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/notifications/service_test.go

@@ -61,6 +61,20 @@ func TestDispatchDueProcessesPendingEmailReminders(t *testing.T) {
 	if len(pending) != 0 {
 		t.Fatalf("expected no pending jobs after dispatch, got %d", len(pending))
 	}
+
+	events, err := repo.ListActivityEvents(context.Background(), "5d6b3551-0a3e-4b86-bdf0-e9df20a47148", 10)
+	if err != nil {
+		t.Fatalf("list activity: %v", err)
+	}
+	for _, event := range events {
+		if event.Action == "reminder.sent" && event.ActorKind == "system" {
+			if event.EntityID == nil || *event.EntityID != created.ID {
+				t.Fatalf("expected entity id %s, got %v", created.ID, event.EntityID)
+			}
+			return
+		}
+	}
+	t.Fatal("expected system reminder.sent activity event")
 }
 
 func TestDispatchDueFailsUnknownChannel(t *testing.T) {