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,

src/utils/s3/object-stream.ts

@@ -60,8 +60,10 @@ const planParts = (parts: ObjectPartSource[], start: number, end: number): Plann
 const streamFromBytes = (bytes: Uint8Array): ReadableStream<Uint8Array> =>
   new Response(bytes).body!;
 
+const TELEGRAM_FETCH_TIMEOUT_MS = 30_000;
+
 const fetchWholePartBytes = async (telegramUrl: string): Promise<Uint8Array> => {
-  const res = await fetch(telegramUrl);
+  const res = await fetch(telegramUrl, { signal: AbortSignal.timeout(TELEGRAM_FETCH_TIMEOUT_MS) });
   if (!res.ok) throw new Error(`Telegram fetch failed: ${res.status}`);
   return new Uint8Array(await res.arrayBuffer());
 };
@@ -77,10 +79,11 @@ const fetchPartBody = async (planned: PlannedPart): Promise<ReadableStream<Uint8
   }
 
   const rangeHeader = `bytes=${planned.relativeStart}-${planned.relativeEnd}`;
-  const res = await fetch(
-    planned.part.telegramUrl,
-    wantsWholePart ? undefined : { headers: { range: rangeHeader } },
-  );
+  const fetchOpts: RequestInit = { signal: AbortSignal.timeout(TELEGRAM_FETCH_TIMEOUT_MS) };
+  if (!wantsWholePart) {
+    fetchOpts.headers = { range: rangeHeader };
+  }
+  const res = await fetch(planned.part.telegramUrl, fetchOpts);
   if (!res.ok) throw new Error(`Telegram fetch failed: ${res.status}`);
   if (wantsWholePart || res.status === 206) return res.body!;