Address review: rename settings to notify_ns_* and add global toggle

Per PR review feedback:
- Rename settings: custom_field_force_notify -> notify_ns_custom_field_name,
  force_notify_port -> notify_ns_port.
- Add notify_ns_all_zones (bool, default False) to enable the feature globally
  for every zone; it coexists with the per-zone custom field — a NOTIFY is sent
  if the global switch is on OR the zone's custom field is set.
- Rename the internal job/scheduler/log strings to the notify_ns naming for
  consistency (SendZoneNSNotify, schedule_notify_ns).
- Update README and CHANGELOG.

CHANGELOG.md

@@ -1,10 +1,10 @@
 ## Unreleased
-- Add `custom_field_force_notify` setting. When set to the name of a boolean custom field on the
-  NetBox DNS Zone object, the plugin sends a NOTIFY directly to a zone's own nameservers whenever
-  the zone has that custom field set to `True` and its SOA serial number changes. Nameserver
-  addresses are resolved from NetBox DNS A/AAAA records within the zone's view and the NOTIFY is
-  signed with the per-view TSIG key. A companion `force_notify_port` setting (default 53) controls
-  the destination port.
+- Add the option to send a NOTIFY directly to a zone's own nameservers when its SOA serial number
+  changes. Enable it per zone via a boolean custom field named by `notify_ns_custom_field_name`,
+  globally for all zones via `notify_ns_all_zones` (default `False`), or both. Nameserver addresses
+  are resolved from NetBox DNS A/AAAA records within the zone's view and the NOTIFY is signed with
+  the per-view TSIG key. A companion `notify_ns_port` setting (default 53) controls the destination
+  port.
 
 ## 1.0.7 - 2026-03-02
 README Change - Moving private keys to global scope since Bind 9.20 view scoped keys have become unreliable and sometimes wouldnt match.