Disable automatic checksums in S3 client for R2 compatibility

botocore 1.36+ defaults to adding CRC32 checksums with aws-chunked
transfer encoding for PutObject, which Cloudflare R2 doesn't support,
causing SignatureDoesNotMatch errors on uploads.

Author: jonathansick

src/docverse/storage/objectstore/_s3.py

@@ -76,7 +76,10 @@ async def open(self) -> None:
             aws_access_key_id=self._access_key_id,
             aws_secret_access_key=self._secret_access_key,
             region_name=self._region or None,
-            config=Config(signature_version="s3v4"),
+            config=Config(
+                signature_version="s3v4",
+                request_checksum_calculation="when_required",
+            ),
         )
         self._client = await self._client_cm.__aenter__()