zone: add PTR record management for DDNS and /zm/update-ptr

[vooon]

Adds PTR record management to Zoneomatic.

New custom endpoint

POST /zm/update-ptr — {"target": "host.example.com.", "addresses": ["192.0.2.55", ...], "mode": "append"|"replace"|"replace-all"}.
Reverse names (in-addr.arpa / ip6.arpa) are calculated from the addresses automatically; mode defaults to replace-all.

Modes (built on the existing matcher machinery, updateRecords gains a replaceAll knob):

• append — insert missing PTR only, never remove anything
• replace — single-record in-place swap, keeps unrelated PTRs on the name
• replace-all — full sync: exact one PTR per address plus removal of stale PTRs pointing at the target

Errs 404 when no matching reverse zone exists, 400 on invalid address/mode.

New CLI option

--ddns-manage-ptr makes /nic/update also manage PTRs for the updated addresses (one PTR per address, stale PTRs cleaned). A missing reverse zone is silently ignored — not an error.

Also

• README: flag, endpoint docs, /nic/update note
• Regenerated doc/openapi.json / doc/openapi.yaml (fuego v0.20.0 output)
• Tests: mode semantics, v4/v6 multi-file updates, no-growth, stale cleanup, disabled/no-zone cases, endpoint HTTP statuses