gatus: turn Signal alerting off estate-wide, with one key

A stopgap, committed so it is visible rather than a surprise: THE PHONE IS
CURRENTLY OFF. Flip `enabled` back to true under the signal provider's
default-alert to turn it on.

Why one key reaches all ~90 endpoints: Gatus merges a provider's default-alert
into each endpoint's alert for every field the endpoint left unset, and
`enabled` is the only alert field no registration playbook sets. So this works
without editing a single endpoint and without unregistering anything. The
dashboard keeps checking and keeps showing red; it just stops sending.

Deliberately NOT done by deleting the `signal:` provider block. That would
strand ~90 endpoints referencing a provider that no longer exists, and throw
away the number and the recipient list with it.

The thing that made this necessary - a 5-minute poll turning one wedged service
into 83 messages in six hours - is fixed properly in the preceding commit. This
key is the blunt instrument that bought time to do that, and it should be
turned back on once the cadence change has been observed for a few days.

Known consequence while it is off: nothing pages. A six-day xmrblocks outage
went unannounced precisely because of this.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ansible/services/gatus/deploy_gatus_playbook.yml

@@ -22,6 +22,21 @@
         number: "{{ signal_number }}"
         recipients: "{{ signal_recipients }}"
         default-alert:
+          # ── SIGNAL ALERTING IS OFF ──────────────────────────────────────
+          # Flip to true to turn the phone back on. Nothing else needs to
+          # change: this ONE key disables every Signal alert in the estate.
+          #
+          # It works because `enabled` is the only alert field no registration
+          # playbook sets. Gatus merges a provider's default-alert into each
+          # endpoint's alert for every field the endpoint left unset, so this
+          # reaches all ~90 of them without editing a single endpoint - and
+          # without unregistering anything. The dashboard keeps checking and
+          # keeps showing red; it just stops sending.
+          #
+          # Deliberately NOT done by deleting the `signal:` provider block:
+          # that would strand ~90 endpoints referencing a provider that no
+          # longer exists, and lose the number and recipients.
+          enabled: false
           # Overridden per group by the registration playbooks; these are the
           # values that apply if a caller sets nothing.
           failure-threshold: 1