Add not found middleware

[Fred0104]

Summary

• add a dedicated notFound middleware for unmatched routes
• register terminal middleware in the documented order: notFound before errorHandler
• enable and update the 404 JSON response test
• fix the test JWT fallback so startup preflight accepts test imports
• update the deprecated TypeScript setting from 6.0 to 5.0, which is accepted by the current TypeScript release

Closes #214.

Validation

• npm test -- errorHandler.spec.ts --runInBand
• Docker clean check (node:22-bookworm): npm ci --ignore-scripts --no-audit --no-fund && npx prisma generate && npm test -- errorHandler.spec.ts --runInBand

Note: npm run build / npm run lint still fail on existing unrelated type errors in leaderboard, scheduler, retention/audit, and retry tests. This PR keeps the change scoped to the 404/error middleware issue.