Add a browserless Chromium service to the compose stack

Run ghcr.io/browserless/chromium as a service in docker-compose, on the internal network only (no Traefik exposure).

• Token auth between the API and the sidecar
• Memory limit and session concurrency cap in the compose config, browserless's built-in queueing replaces the hand-rolled batch-of-6 logic in the scraper
• Add it to both the prod compose file and the dev override so the scraper works locally too