#523 - Quick wins: runtime stability, CI alignment, and config/test hygiene

Parent: #523

## Objective
Deliver low-effort, high-impact updates that reduce immediate operational risk and improve signal quality.

## In scope
- Remove error rethrow after HTTP response in `src/index.ts`
- Ensure fatal server bootstrap failures exit non-zero in `src/server.ts`
- Align GitHub Actions runtime with supported Node version and update action versions
- Update Vitest include patterns to cover JS + TS specs
- Refresh docs/tooling references that are clearly outdated

## Acceptance criteria
- API error middleware does not rethrow after sending response
- Startup failure produces non-zero process exit
- CI runs on supported Node version and passes
- Test runner discovers both `*.spec.js` and `*.spec.ts`
- Updated docs reflect current commands and runtime expectations

## Notes
Keep this issue focused on minimal-risk, behavior-preserving changes.