feat: wire brain service to platform gateway

- Gateway proxies /api/brain/* to brain-api:8200/api/* via pangolin network
- User identity injected via X-Gateway-User-Id header
- Brain app registered in gateway database (sort_order 9)
- Added to GATEWAY_KEY_SERVICES for dashboard integration
- Tested: health, config, list, create all working through gateway

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -61,6 +61,7 @@ services:
       - BUDGET_BACKEND_URL=http://budget-service:3001
       - TASKS_BACKEND_URL=http://tasks-service:8098
       - TASKS_SERVICE_API_KEY=${TASKS_SERVICE_API_KEY}
+      - BRAIN_BACKEND_URL=http://brain-api:8200
       - QBITTORRENT_HOST=${QBITTORRENT_HOST:-192.168.1.42}
       - QBITTORRENT_PORT=${QBITTORRENT_PORT:-8080}
       - QBITTORRENT_USERNAME=${QBITTORRENT_USERNAME:-admin}