feat(provider): feno (feno.no)

[MrErikCodes]

Adds a dedicated provider for FENO, a Norwegian .no registrar with hosted DNS.

FENO's dynamic DNS endpoint (https://api.feno.no/v1/nic/update) speaks the standard dyndns2 protocol: HTTP Basic auth where the API key is the password (username ignored), hostname=, myip=/myipv6=, plain-text good/nochg/badauth/nohost/notfqdn/numhost/badagent/abuse/dnserr/911 responses. Public documentation: https://github.com/mrerikcodes/feno-api/blob/main/docs/DDNS.md

It is usable today through the custom provider, but that only allows one query key for both IP families. The dedicated provider sends myip for IPv4 and myipv6 for IPv6, so A and AAAA records are updated independently, and it maps every documented response token onto the existing internal/provider/errors values (modelled on the noip/dyn providers).

Configuration:

{
  "provider": "feno",
  "domain": "example.no",
  "owner": "home",
  "api_key": "feno_live_...",
  "ip_version": "ipv4"
}

Changes (same set of files as the recent hostinger / gigahostno provider PRs):

• internal/provider/providers/feno/provider.go — provider implementation
• internal/provider/providers/feno/provider_test.go — unit tests for settings validation and response parsing
• internal/provider/constants/providers.go + internal/provider/provider.go — registration
• docs/feno.md — documentation page (from the docs/example.md template)
• README.md — provider list entries

Checks: go build ./..., go vet ./..., go test ./... (incl. the readme package test) all pass; golangci-lint run (v2.11.4, the version pinned in the Dockerfile) reports no issues for the new package.

I maintain the FENO API side, so I am happy to adjust anything on either end.

Live run against a real FENO zone (ddnstest.2002.no, the exact request the provider builds, HTTP Basic with the key as password):

?hostname=ddnstest.2002.no&myip=203.0.113.7      -> good 203.0.113.7
?hostname=ddnstest.2002.no&myip=203.0.113.7      -> nochg 203.0.113.7
?hostname=ddnstest.2002.no&myipv6=2001:db8::1    -> good 2001:db8::1