fix: add ETag and headers to 304 Not Modified responses

AWS SDK requires ETag header in 304 responses. Without it, the SDK
throws UnknownError despite receiving a valid 304 status code.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/utils/s3/auth.ts

@@ -297,7 +297,7 @@ export const verifySignature = async (
   const hashedCanonicalRequest = await sha256Hex(canonicalRequest);
 
   // M1: Fall back to Date header if x-amz-date is missing
-  const amzDate = headers['x-amz-date'] || headers['date'] || '';
+  const amzDate = headers['x-amz-date'] || headers.date || '';
 
   // H5: Validate request freshness (clock skew / replay protection)
   if (amzDate) {