feat(infra): Nix build for scraper, GitHub Actions workflow

- Build scraper (Rust) with Nix — cargo build --release
- Create scraper systemd unit (port 4091), env from Docker config
- Fix HOME/CARGO_HOME for Rust/cargo in Nix sandbox
- Update Traefik apps.yaml: scraper -> host.docker.internal:4091
- Add iptables rules for port 4091 (Docker->host)
- Add GitHub Actions workflow: nix-build.yml (determinate-nix + deploy)
- Save iptables rules persistently

infra/traefik/dynamic/apps.yaml

@@ -59,7 +59,7 @@ http:
     scraper-service:
       loadBalancer:
         servers:
-          - url: 'http://scraper-api:4091'
+          - url: 'http://host.docker.internal:4091'
 
     tools-service:
       loadBalancer: