Network isolation: egress firewall + named bridge

Adds the host-level egress firewall recommended by the upstream
DeerFlow team's "run in a VLAN" guidance, adapted to a Fritzbox-only
home network where LAN VLANs are not available.

- docker/docker-compose.override.yaml: pins the upstream deer-flow
  Docker network to a stable Linux bridge name br-deerflow so the
  firewall can address it without guessing Docker's auto-generated
  br-<hash>. Used as a -f overlay on top of the upstream compose file.

- scripts/deerflow-firewall.sh: idempotent up/down/status wrapper that
  installs DOCKER-USER iptables rules. Allowlist for 10.67.67.1 (Searx)
  and 10.67.67.2 (XTTS/Whisper/Ollama-local), hard block for
  192.168.3.0/24 (home LAN), 10.0.0.0/8, 172.16.0.0/12. Stateful return
  rule keeps inbound LAN access to published ports working.

- scripts/deerflow-firewall.nix: NixOS module snippet defining a
  systemd unit ordered After=docker.service so the rules survive
  dockerd restarts and follow its lifecycle. Copy into
  configuration.nix and nixos-rebuild switch.

- HARDENING.md: new section 2.7 "Network isolation (egress firewall)"
  with allow/block tables, bring-up steps, and smoke-test commands.

Guarantees: rules match on -i br-deerflow, so if the bridge does not
exist, the rules are no-ops and do not affect any other container
(paperclip, telebrowser, openclaw-gateway, ...). Stopping the
container leaves the rules in place but inert; stopping the systemd
unit removes them.

docker/docker-compose.override.yaml

@@ -0,0 +1,27 @@
+# DeerFlow hardening overlay.
+#
+# This override turns the upstream `deer-flow` bridge network into a
+# stably-named Linux bridge (`br-deerflow`) so that the host-level
+# egress firewall (deerflow-firewall.service) can reference it without
+# guessing Docker's auto-generated `br-<hash>` name.
+#
+# Use it together with the upstream compose file:
+#
+#     cd deerflow-factory
+#     docker compose \
+#         -f deer-flow/docker/docker-compose.yaml \
+#         -f docker/docker-compose.override.yaml \
+#         up -d
+#
+# After the network is created (or recreated), the firewall rules
+# installed by deerflow-firewall.service automatically take effect on
+# the new bridge — no further action required.
+#
+# If you ever rename or delete the bridge, restart the firewall unit:
+#     systemctl restart deerflow-firewall.service
+
+networks:
+  deer-flow:
+    driver: bridge
+    driver_opts:
+      com.docker.network.bridge.name: br-deerflow