Fix S3 client to support IRSA credentials (#54)

Only pass explicit credentials to boto3 if they're actually set.
This allows the default credential chain (including IRSA web identity
tokens) to be used when no access key is configured.

Also adds CHANGELOG entries for AWS services configuration.

backend/app/storage.py

@@ -242,15 +242,19 @@ class S3Storage:
             },
         )
 
-        self.client = boto3.client(
-            "s3",
-            endpoint_url=settings.s3_endpoint if settings.s3_endpoint else None,
-            region_name=settings.s3_region,
-            aws_access_key_id=settings.s3_access_key_id,
-            aws_secret_access_key=settings.s3_secret_access_key,
-            config=config,
-            verify=settings.s3_verify_ssl,  # SSL/TLS verification
-        )
+        # Build client kwargs - only include credentials if explicitly provided
+        # This allows IRSA/IAM role credentials to be used when no explicit creds are set
+        client_kwargs = {
+            "endpoint_url": settings.s3_endpoint if settings.s3_endpoint else None,
+            "region_name": settings.s3_region,
+            "config": config,
+            "verify": settings.s3_verify_ssl,
+        }
+        if settings.s3_access_key_id and settings.s3_secret_access_key:
+            client_kwargs["aws_access_key_id"] = settings.s3_access_key_id
+            client_kwargs["aws_secret_access_key"] = settings.s3_secret_access_key
+
+        self.client = boto3.client("s3", **client_kwargs)
         self.bucket = settings.s3_bucket
         # Store active multipart uploads for resumable support
         self._active_uploads: Dict[str, Dict[str, Any]] = {}