Reduce R2 upload concurrency to 10

20 concurrent workers triggered R2 rate limiting causing mass retries.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: retlehs

internal/deploy/r2.go

@@ -91,7 +91,7 @@ func SyncToR2(ctx context.Context, cfg config.R2Config, buildDir, buildID, previ
 	// Upload all files under the release prefix (parallel).
 	var uploaded, copied atomic.Int64
 	g, gCtx := errgroup.WithContext(ctx)
-	g.SetLimit(20)
+	g.SetLimit(10)
 
 	for _, f := range files {
 		f := f