refactor(backup): type hints sur FakeStorage (#89)

tests/backup/fakes.py

@@ -1,15 +1,15 @@
 class FakeStorage:
-    def __init__(self):
+    def __init__(self) -> None:
         self.objects: dict[str, bytes] = {}
 
-    def upload_bytes(self, key, data):
+    def upload_bytes(self, key: str, data: bytes) -> None:
         self.objects[key] = data
 
-    def download_bytes(self, key):
+    def download_bytes(self, key: str) -> bytes:
         return self.objects[key]
 
-    def list_keys(self, prefix):
+    def list_keys(self, prefix: str) -> list[str]:
         return [k for k in self.objects if k.startswith(prefix)]
 
-    def delete(self, key):
+    def delete(self, key: str) -> None:
         self.objects.pop(key, None)