feat: push notifications for DMs

- DM handler now sends push notifications to other conversation members
- Cleared stale push subscriptions (old broken VAPID keys)
- Auto-subscribe will re-create them on next page load

cmd/server/main.go

@@ -212,7 +212,7 @@ func main() {
 			})
 
 			// Direct messages
-			dmHandler := dm.NewHandler(database.DB, hub, logger)
+			dmHandler := dm.NewHandler(database.DB, hub, pushHandler, logger)
 			r.Route("/conversations", func(r chi.Router) {
 				dmHandler.RegisterRoutes(r)
 			})