fix: round 2 S3 audit — CRITICAL SigV4 payload hash bug, timeouts, Content-MD5/Length validation

CRITICAL:
- SigV4 canonical request used sha256Hex('') instead of x-amz-content-sha256
  header value — every PUT/POST with body would fail 403. Now uses the
  signed header value for canonical request, verifyBodyHash after streaming
  for integrity.

HIGH:
- Add 30s AbortSignal.timeout to all Telegram CDN fetches in object-stream.ts
  (previously could hang indefinitely, exhausting connection pool)

MEDIUM:
- Content-MD5 validation: compute and compare when header is present
- Content-Length validation: reject if actual body size != header
- max-keys=0 clamping: enforce minimum of 1 per S3 spec

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

src/utils/s3/auth.ts

@@ -282,10 +282,11 @@ export const verifySignature = async (
     return { isValid: false, credential: null, errorCode: 'NotImplemented' };
   }
 
-  // H4: Compute hash from actual body instead of trusting header blindly.
-  // For streaming bodies (body === null), we cannot hash at this point —
-  // the caller (controller) must verify body hash after streaming.
-  const hashedPayload = await getHashedPayload(body);
+  // CRITICAL: Use the x-amz-content-sha256 header value in the canonical
+  // request because that's what the client signed. The actual body hash is
+  // verified by verifyBodyHash() after streaming, ensuring integrity without
+  // breaking SigV4.
+  const hashedPayload = contentSha256 || (await getHashedPayload(body));
 
   const canonicalRequest = buildCanonicalRequest(
     method,