fix(api): configure root_path for OpenAPI docs behind reverse proxy

Add ROOT_PATH environment variable support so FastAPI generates correct
URLs for Swagger UI when served behind nginx at /api. This fixes the
"invalid version field" error when accessing /api/docs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docker-compose.yml

@@ -38,6 +38,7 @@ services:
       DATABASE_URL: postgresql://postgres:postgres@postgres:5432/sparc
       USE_CACHE: "true"
       JWT_SECRET: ${JWT_SECRET:-sparc-secret-key-change-in-production}
+      ROOT_PATH: /api
     ports:
       - "8000:8000"
     depends_on: