fix: upgrade Nodemailer to 7.0.3 and resolve Express 5 route conflicts

- Upgraded Nodemailer 6.10.1 → 7.0.3 (fully compatible with SMTP usage)
- Temporarily keep Express 4.21.2 due to path-to-regexp v8 breaking changes
- Removed duplicate threshold routes from configApi.js (moved to thresholdRoutes.js)
- Express 5 upgrade will require route pattern updates for compatibility

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: rcourtman