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.

README.md

@@ -98,22 +98,31 @@ list of clients to be notified on zone changes. Default is 24 hours.
 ```
 Switch to using TCP instead of UDP. Defaults to False
 
-#### Forced NOTIFY to a zone's nameservers
+#### NOTIFY to a zone's own nameservers
 In addition to notifying clients that queried the transfer endpoint, the plugin can send a NOTIFY
 directly to a zone's own nameservers whenever that zone's SOA serial number changes. This is useful
-when you want specific zones to have their nameservers pull updates immediately, independent of the
-`notify_clients` mechanism.
-
-The feature is driven by a boolean custom field on the NetBox DNS Zone object. Create a boolean
-custom field (e.g. `dns_bridge_force_notify_ns`) that applies to `netbox_dns > zone`, then point the
-plugin at it:
-```
-'custom_field_force_notify': 'dns_bridge_force_notify_ns'
-```
-When this setting is present, every time a zone whose custom field is set to `True` changes its SOA
-serial number, the plugin schedules a background NOTIFY to each of the zone's configured nameservers.
+when you want nameservers to pull updates immediately, independent of the `notify_clients` mechanism.
 Only a change of the SOA serial triggers this — other zone edits that leave the serial untouched do
-not. The setting is unset by default, which disables the feature entirely.
+not.
+
+This can be enabled per zone, globally, or both. If neither of the settings below is configured, the
+feature is disabled entirely.
+
+**Per zone (custom field).** Create a boolean custom field (e.g. `dns_bridge_notify_ns`) that applies
+to `netbox_dns > zone`, then point the plugin at it:
+```
+'notify_ns_custom_field_name': 'dns_bridge_notify_ns'
+```
+A NOTIFY is then sent for any zone whose custom field is set to `True`. This lets you toggle the
+behavior per zone straight from the NetBox UI, without changing the plugin config.
+
+**Globally (all zones).**
+```
+'notify_ns_all_zones': True
+```
+When enabled, a NOTIFY is sent for every zone on serial change, regardless of the custom field.
+Defaults to `False`. The two options coexist: a NOTIFY is sent if the global switch is on **or** the
+zone's custom field is set.
 
 The nameserver addresses are resolved from NetBox DNS itself by looking up active A/AAAA records
 (within the zone's view) matching each nameserver's hostname. The NOTIFY messages are signed with the
@@ -121,9 +130,9 @@ same per-view TSIG key configured under `tsig_keys`, so the receiving nameserver
 signed with that key (e.g. BIND's `allow-notify { key "..."; };`).
 
 ```
-'force_notify_port': 53
+'notify_ns_port': 53
 ```
-Destination port used for these forced NOTIFY messages. Defaults to 53.
+Destination port used for these NOTIFY messages. Defaults to 53.
 
 As with the client NOTIFY feature, this uses NetBox's background job system, so at least one
 `rq-worker` service must be running.