Add a generic NAT table: port forward, source NAT, masquerade

router.firewall.nat replaces the dnat/toPort/hairpin fields of the filter
rules. Filter rules gain natOnly, and may use wan with log and rate limit
in the web UI. The Firewall page edits the NAT table and can add the
matching allow rule. Settings schema 1 (migrations/1.jq) converts old
forwards.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

migrations/1.jq

@@ -0,0 +1,21 @@
+# Port forwards move from `firewall.rules` (dnat, toPort, hairpin) to
+# `firewall.nat`; the allow rule that went with them stays as a rule of its
+# own, limited to translated connections (natOnly). The rest of a rule
+# (rateLimit, log, sources) stays on the allow rule.
+def natRow:
+  {type: "dnat", from: .from, to: .to, protocol: .protocol, ports: .ports, toAddress: .destination}
+  + (if .toPort != null then {toPort: .toPort} else {} end)
+  + (if ((.sources // []) | length) > 0 then {sources: .sources} else {} end)
+  + (if .hairpin == true then {hairpin: true} else {} end);
+
+def allowRule:
+  del(.dnat, .hairpin, .toPort)
+  + {natOnly: true}
+  + (if .toPort != null then {ports: [.toPort]} else {} end);
+
+if ((.router.firewall.rules // []) | any(.dnat == true or .hairpin != null or .toPort != null)) then
+  .router.firewall.nat = ((.router.firewall.nat // []) + [.router.firewall.rules[] | select(.dnat == true) | natRow])
+  | .router.firewall.rules |= map(if .dnat == true then allowRule else del(.dnat, .hairpin, .toPort) end)
+else
+  .
+end