Remove old root-level Express routes

## Parent PRD

- Parent #1618

## What to build

Remove the old non-prefixed Express routes (`/gremlin`, `/sparql`, `/openCypher`, `/summary`, `/pg/statistics/summary`, `/rdf/statistics/summary`, `/logger`, `/status`, `/defaultConnection`) that were kept for backward compatibility during the transition. After this, the only API path is `/api/*`. Clean up any remaining references to the old URL patterns.

This is the final cleanup slice that completes the migration.

**Type: HITL** — requires review before completion.

## Acceptance criteria

- [ ] Old root-level routes are removed from the Express server
- [ ] Only `/api/*` routes remain for API endpoints
- [ ] No client code references old root-level API paths
- [ ] Tests verify old routes return 404
- [ ] All existing tests pass with the new route structure

## Blocked by

- Blocked by #1621
- Blocked by #1623
- Blocked by #1626

## User stories addressed

- User story 9
- User story 13