implement S3 garage on the backend

apps/backend/app/s3.py

@@ -0,0 +1,11 @@
+import os
+import boto3
+
+s3_client = boto3.client(
+    "s3",
+    endpoint_url=os.getenv("GARAGE_ENDPOINT_URL", "http://127.0.0.1:3900"),
+    aws_access_key_id=os.getenv("GARAGE_ACCESS_KEY"),
+    aws_secret_access_key=os.getenv("GARAGE_SECRET_KEY"),
+)
+
+BUCKET_NAME = os.getenv("GARAGE_BUCKET_NAME", "nom-de-ton-bucket")