fix(server): use Buffer.length for timing-safe HMAC comparison and document header fallback

Compare Buffer byte lengths instead of string character lengths before
timingSafeEqual to avoid potential mismatch with multi-byte input.
Add comment explaining the hubSignatureHeader ?? signatureHeader fallback.

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

Author: antonio-mello-ai

server/src/services/routines.ts

@@ -1272,16 +1272,21 @@ export function routineService(db: Db, deps: { heartbeat?: IssueAssignmentWakeup
       } else if (trigger.signingMode === "github_hmac") {
         const secretValue = await resolveTriggerSecret(trigger, routine.companyId);
         const rawBody = input.rawBody ?? Buffer.from(JSON.stringify(input.payload ?? {}));
+        // Accept X-Hub-Signature-256 (GitHub/Sentry) or fall back to the
+        // generic X-Paperclip-Signature header so operators can use github_hmac
+        // mode with either header convention.
         const providedSignature = (input.hubSignatureHeader ?? input.signatureHeader)?.trim() ?? "";
         if (!providedSignature) throw unauthorized();
         const expectedHmac = crypto
           .createHmac("sha256", secretValue)
           .update(rawBody)
           .digest("hex");
         const normalizedSignature = providedSignature.replace(/^sha256=/, "");
+        const normalizedBuf = Buffer.from(normalizedSignature);
+        const expectedBuf = Buffer.from(expectedHmac);
         const valid =
-          normalizedSignature.length === expectedHmac.length &&
-          crypto.timingSafeEqual(Buffer.from(normalizedSignature), Buffer.from(expectedHmac));
+          normalizedBuf.length === expectedBuf.length &&
+          crypto.timingSafeEqual(normalizedBuf, expectedBuf);
         if (!valid) throw unauthorized();
       } else if (trigger.signingMode === "bearer") {
         const secretValue = await resolveTriggerSecret(trigger, routine.companyId);