Move Express API routes under /api/ prefix

## Parent PRD

- Parent #1618

## What to build

Mount all existing Express proxy routes (`/gremlin`, `/sparql`, `/openCypher`, `/summary`, `/pg/statistics/summary`, `/rdf/statistics/summary`, `/logger`, `/status`, `/defaultConnection`) under an `/api/` prefix. Keep the old root-level routes working alongside the new ones for backward compatibility during the transition.

This is the foundational server-side change that all other slices depend on.

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

## Acceptance criteria

- [ ] All API endpoints respond under `/api/` prefix (e.g., `POST /api/gremlin`, `POST /api/sparql`)
- [ ] Old root-level routes still work (e.g., `POST /gremlin` still responds)
- [ ] Tests verify the new `/api/*` paths respond correctly
- [ ] No changes to client code in this slice

## Blocked by

None - can start immediately

## User stories addressed

- User story 14

Locate the Express route registration for /gremlin, /sparql, /openCypher, /summary, the statistics routes, /logger, /status, and /defaultConnection, then find the existing tests for these endpoints. Run those tests first and add coverage for each /api/* path while confirming the original root-level paths still respond. Done means all listed routes work under both prefixes without client changes.

Written by the indexing model from the issue text.