chore: fix eslint errors introduced in phase 4 backend

packages/backend/convex/githubWebhookVerify.ts

@@ -17,7 +17,7 @@ export const verifyGithubSignature = async (
   rawBody: string,
   signatureHeader: string | null,
 ): Promise<boolean> => {
-  if (!signatureHeader || !signatureHeader.startsWith('sha256=')) return false;
+  if (!signatureHeader?.startsWith('sha256=')) return false;
   // Web Crypto's importKey throws on a zero-length HMAC key; guard so the
   // verifier never throws on attacker-controlled input (returns false instead).
   if (!secret) return false;