fix: boto3 1.35+ Garage S3 checksum regression

Add request_checksum_calculation=when_required to boto3 Config to
prevent STREAMING-* sha256 header that Garage S3 rejects.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server.py

@@ -49,7 +49,11 @@ def _s3_upload_png(png: bytes, job_id: str) -> str | None:
             aws_access_key_id=_S3_ACCESS,
             aws_secret_access_key=_S3_SECRET,
             region_name="garage",
-            config=Config(signature_version="s3v4"),
+            config=Config(
+                signature_version="s3v4",
+                request_checksum_calculation="when_required",
+                response_checksum_validation="when_required",
+            ),
         ).put_object(Bucket=_S3_BUCKET, Key=key, Body=png, ContentType="image/png")
         return f"{_S3_PUBLIC_BASE}/{key}"
     except Exception: