feat: support custom notifications (#19751)

## Description

Adds support for sending an ad‑hoc custom notification to the
authenticated user via API and CLI. This is useful for surfacing the
result of scripts or long‑running tasks. Notifications are delivered
through the configured method and the dashboard Inbox, respecting
existing preferences and delivery settings.

## Changes

* New notification template: “Custom Notification” with a label for a
custom title and a custom message.
* New API endpoint: `POST /api/v2/notifications/custom` to send a custom
notification to the requesting user.
* New API endpoint: `GET /notifications/templates/custom` to get custom
notification template.
* New CLI subcommand: `coder notifications custom <title> <message>` to
send a custom notification to the requesting user.
* Documentation updates: Add a “Custom notifications” section under
Administration > Monitoring > Notifications, including instructions on
sending custom notifications and examples of when to use them.

Closes: https://github.com/coder/coder/issues/19611

cli/testdata/coder_notifications_--help.golden

@@ -12,7 +12,7 @@ USAGE:
   from
   dispatching messages in case of the target outage (for example: unavailable
   SMTP
-  server or Webhook not responding).:
+  server or Webhook not responding):
   
        $ coder notifications pause
   
@@ -22,11 +22,17 @@ USAGE:
   
     - Send a test notification. Administrators can use this to verify the
   notification
-  target settings.:
+  target settings:
   
        $ coder notifications test
+  
+    - Send a custom notification to the requesting user. Sending notifications
+  targeting other users or groups is currently not supported:
+  
+       $ coder notifications custom "Custom Title" "Custom Message"
 
 SUBCOMMANDS:
+    custom    Send a custom notification
     pause     Pause notifications
     resume    Resume notifications
     test      Send a test notification

cli/testdata/coder_notifications_custom_--help.golden

@@ -0,0 +1,9 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder notifications custom <title> <message>
+
+  Send a custom notification
+
+———
+Run `coder --help` for a list of global options.