refacto: rename app into backend, to prepare for frontend dev

server/backend/ports/message_broker.py

@@ -0,0 +1,10 @@
+from abc import ABC, abstractmethod
+from typing import Callable
+
+from domain.entities import UplinkEvent
+
+class MessageBroker(ABC):
+    @abstractmethod
+    def start(self, on_uplink: Callable[[UplinkEvent], None]) -> None:
+        """Démarre l'écoute et appelle on_uplink(UplinkEvent) à chaque message"""
+        ...