Add dynamic DNS: keep A records pointed at the current public IP

For deployments on WAN DHCP, operators can declare A records in the DNS section
that the app updates to this box's current public IP every 4 hours (and
immediately on create).

- utils/public_ip.js: getPublicIp() queries external echo services (ipify +
  fallbacks, configurable) with pure isIPv4/extractIp helpers.
- utils/dns_records.js: pure planARecordUpdate() reconciliation decision.
- models/dns_provider.js: Domain.upsertARecord(name, ip) — provider-agnostic
  upsert via getRecords + deleteRecordById + createRecord (createRecord alone is
  not a reliable cross-provider upsert). Apex ('@') handling added to each
  provider (CloudFlare uses the domain name, Porkbun an empty name, DigitalOcean
  '@') via a new DnsApi.apexName().
- models/dynamic_record.js: DynamicRecord model (deterministic id per host,
  apply()/refreshAll()), registered + ModelPs-wrapped for live UI updates.
- services/dynamic_dns.js + conf: 4h scheduler mirroring host_scheduler.
- routes/dns.js: /dynamic CRUD + /dynamic/ip, gated to domain managers/admins.
- views/dns.ejs: "Dynamic A Records (WAN IP)" card with add form + list.
- test/unit/dynamic_record.test.js: public-IP parsing + reconciliation logic.

Verified end-to-end against a live Porkbun domain (create, idempotent, IP-change,
cleanup) plus unit suite (111 pass).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nodejs/models/dns_provider/common.js

@@ -61,6 +61,14 @@ class DnsApi{
 		if(!['A', 'MX', 'CNAME', 'ALIAS', 'TXT', 'NS', 'AAAA', 'SRV', 'TLSA', 'CAA', 'HTTPS', 'SVCB'].includes(type)) throw new Error('PorkBun API: Invalid type passed')
 	}
 
+	// How this provider names an apex (root-of-domain) record. Callers pass the
+	// sentinel '@' for apex; each provider maps it to its own convention. Default
+	// is '@' (DigitalOcean). CloudFlare uses the full domain name; Porkbun uses
+	// an empty name. Sub-domain records are passed through unchanged.
+	apexName(domainName){
+		return '@';
+	}
+
 
 	/*
 	The API and the generic class interface have different opinions of what keys