Add Docker + nginx deployment for homelab

- Multi-stage Dockerfile: Node builds static assets, nginx serves them
- nginx proxies /api/* to football-data.org and injects X-Auth-Token
  server-side via FOOTBALL_API_KEY env var (key never in browser bundle)
- docker-compose.yml exposes the app on port 3000
- Extract apiFetch() helper: sends key in dev (Vite proxy), skips in prod
- .env.example updated with both dev and production key vars

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -1 +1,5 @@
+# Used by docker-compose (nginx injects it server-side — never exposed to browser)
+FOOTBALL_API_KEY=your_api_key_here
+
+# Only needed for local development (npm run dev)
 VITE_FOOTBALL_API_KEY=your_api_key_here