feat: add server-to-server auth for persisting auto-translated node content

## Context

PR #257 adds ISR + async auto-translation of node descriptions via OpenAI. Currently, translations are only cached in Vercel CDN (`s-maxage=3600`) but **not persisted to the database** via `POST /nodes/{nodeId}/translations`.

## Problem

The translations API requires Firebase user auth (`Bearer `). The server-side API route (`/api/translate-node`) has no user session, so it cannot authenticate with the backend to persist translations.

## Proposed Solution

Add a **service account / API key** mechanism to the registry backend that allows the frontend server to write translations without a user session.

Options:
1. **Backend API key**: A dedicated `X-API-Key` or `Authorization: Bearer ` for server-to-server translation writes
2. **Firebase service account**: Use Firebase Admin SDK with a service account to generate tokens server-side
3. **Separate endpoint**: `POST /nodes/{nodeId}/translations/auto` that accepts a shared secret instead of user auth

## Why this matters

Without persistence, every ISR revalidation re-translates the same content via OpenAI, wasting API credits. With persistence:
- Translate once → stored in `node.translations[locale]` in the DB
- Subsequent ISR builds read the stored translation (no OpenAI call)
- Publishers can override auto-translations via the translation editor (PR #179)

## Related

- PR #257 — ISR + auto-translation (current implementation)
- PR #179 — Node translation editor UI
- `POST /nodes/{nodeId}/translations` — existing backend endpoint
- `node.translations` field — already in the Node schema

Start by reading the /api/translate-node route, the existing POST /nodes/{nodeId}/translations endpoint, and the node.translations schema field. Compare the proposed authentication options and related PRs #257 and #179; done means server-side translation writes persist successfully without a Firebase user session and later ISR requests can reuse them.

Written by the indexing model from the issue text.