[codex] Add Caddy agent ingress allowlist

[wingsrabbit]

Summary

• Bump ONC to v1.12.
• Add nodes.agent_allowed_cidrs as the configured agent ingress allowlist, with IPv4/IPv6 IP/CIDR validation and normalization.
• Move gunicorn to 127.0.0.1:18080; Caddy now owns public 80/443/8080 and protects 8080 with a remote_ip allowlist.
• Regenerate and hot-reload Caddy when nodes are added, edited, deleted, disabled/enabled, or their allowed source CIDRs change.
• Update the admin Nodes UI to require/display allowed sources and warn when the observed public_ip does not match.

Security Semantics

• Caddy provides a coarse 8080 IP gate using the enabled nodes' allowlist union.
• Flask still binds each agent token to that node's own agent_allowed_cidrs; a request from node A's IP using node B's token is rejected with 403.
• Disabled nodes are excluded from the Caddy allowlist and their tokens are rejected by Flask with 403.
• Invalid agent tokens still return 401 when a request reaches Flask.

Migration

• Existing databases get a new nodes.agent_allowed_cidrs column.
• During db.init_db(), nodes with an empty allowlist inherit their existing public_ip as /32 or /128 when valid.
• Nodes without a usable public_ip remain blank and must be filled by an admin before they can pass the 8080 allowlist.
• public_ip remains an observation from the latest report; it is no longer used as ingress configuration.

Compatibility

• Requests to 8080 from non-allowlisted IPs are rejected at Caddy with 403 and do not reach gunicorn.
• 80/443 web/admin/public access is not IP-restricted by the agent allowlist.
• deploy/update.sh now also copies supervisord.conf and entrypoint.sh so existing containers switch to the internal gunicorn + Caddy 8080 model during standard update.
• The standard update health check now runs inside nc-center with docker exec because 18080 is container-internal.
• manage.py create-node now hot-reloads Caddy with the candidate allowlist before writing the node.

Validation

• python3 -m unittest discover -s backend -p 'test_*.py' passed: 10 tests.\n- python3 -m compileall backend agent passed.\n- npm run build in frontend/ passed; Vite reported the existing large chunk warning.\n- Generated Caddyfile sample includes :8080, remote_ip, guarded reverse_proxy 127.0.0.1:18080, and fallback respond 403.\n- Local machine does not have caddy or Docker installed, so real caddy validate was not run locally.\n\n## Rollback\n- Revert this PR and deploy the previous release.\n- If already deployed, run the standard updater after revert, or rerun install-center.sh from the previous release to restore the old runtime wiring.\n- Database rollback is not required for app startup: the added column is additive. If a strict schema rollback is desired, back up nc.sqlite first and drop nodes.agent_allowed_cidrs manually only after reverting code.