Fix for "Unauthorized" errors in Convex mutations called from Next.js API routes when using
internal API secrets for authentication. Use when: (1) Convex mutation throws "Unauthorized"
even though secret is set in .env.local, (2) webhook handlers fail with 500 status and
"Unauthorized" in Convex function, (3) internal API calls work locally in Next.js but fail
when calling Convex mutations. The fix is to set the secret in BOTH Next.js .env.local AND
Convex environment variables.