[ HumanjavaEnterprises/nostr-auth-middleware ] fix: allow test environment to run without JWT_SECRET env var

Config now falls back to a placeholder JWT secret when NODE_ENV=test
or VITEST is set, preventing module import failures during testing.
Production environments still require JWT_SECRET to be set.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
https://github.com/HumanjavaEnterprises/nostr-auth-middleware/commit/ccc5645470f0e7dfa33d3f86bc524480346f054b