fix(worker): set ACB_B2_REGION to empty string for ARMOR/B2 S3-compatible API

The AWS SDK rejects 'us-east-1' as a region when using a custom S3-compatible
endpoint (ARMOR proxy wrapping Backblaze B2). The B2 client code already
hardcodes config.WithRegion('auto') which is correct for S3-compatible APIs.

This fixes the 'Invalid region: region was not a valid DNS name' error that
was preventing replay uploads to B2.

manifests/acb-worker-deployment.yml

@@ -77,7 +77,7 @@ spec:
                   name: acb-armor-credentials
                   key: auth-secret-key
             - name: ACB_B2_REGION
-              value: "us-east-1"
+              value: ""
             - name: ACB_METRICS_ADDR
               value: ":9090"
           resources: