feat: webhook alerts for monitor + domain events (merge plan M2)

Generic webhook notification delivery configured via the
notify_webhook settings key — no new schema. Fires on monitor
up/down transitions (pending stays silent), domain expiry,
domain expiring, and TLS cert expiring; dispatch is async and
failures are logged, never fatal.

POST /api/notify/test verifies configuration; Settings dialog
gains a webhook URL field with save + send-test. Payload carries
event/name/text/content/message so Slack, Discord, ntfy, and
custom receivers all work.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

openapi.yaml

@@ -540,6 +540,17 @@ paths:
                 additionalProperties:
                   $ref: "#/components/schemas/Status"
 
+  /api/notify/test:
+    post:
+      operationId: notifyTest
+      summary: Send a test notification to the configured webhook
+      description: Requires the `notify_webhook` settings key to hold an http(s) URL.
+      responses:
+        "200":
+          description: Test notification dispatched
+        "400":
+          $ref: "#/components/responses/BadRequest"
+
   /api/settings:
     get:
       operationId: getSettings