Disable zones instead of deleting them

A disabled zone stays in Jarl with its records, options and history,
still editable, but named doesn't load it; enabling it serves it as it is
then. Signed zones, and zones another view shares, can't be disabled.
jarl zone disable|enable, the zone page's Disable/Enable button.
Database schema 7.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/validation.md

@@ -57,6 +57,8 @@ showing them.
 | `zone_missing_option` | error | Secondary, stub and mirror zones need `primaries` |
 | `zone_file_required` / `zone_file_invalid` / `zone_file_managed` | error | Hint zones need a file. Forward zones have none. Primary zone files are Jarl's |
 | `zone_inview_invalid`, `zone_inview_option` | error | `in-view` zones only inside views, and only with `forward`/`forwarders` |
+| `zone_inview_disabled` | error | An `in-view` zone can't share a disabled zone |
+| `zone_disabled_signed` | error | A signed zone can't be disabled: turn signing off first |
 | `zone_records_unmanaged` | error | Only primary zones can have records in Jarl |
 | `zone_serial_policy` | error | `date`, `increment` or `unixtime` |
 | `zone_no_forwarders`, `zone_hint_name`, `zone_name_slash` | info / warning | Unusual but legal setups |