feat(observability): add structured audit log channel for API and webhook events

Introduce a dedicated `audit` log channel (daily rotation, configurable retention via
LOG_AUDIT_DAYS) and a small `auditLog()` / `auditLogWebhookFailure()` helper used to
record state-changing API operations and webhook events.

Instrumented:

- API mutation endpoints (create / update / delete / start / stop / restart) across
  applications, services, databases (incl. backups, env vars, storage), servers,
  projects + environments, scheduled tasks, private keys, GitHub apps, cloud provider
  tokens, Hetzner server provisioning, instance enable/disable.
- Webhook signature verification outcomes for GitHub, GitLab, Bitbucket, Gitea and
  Stripe, plus the Sentinel push endpoint.
- Authentication and authorization outcomes via the global exception handler and
  the `ApiAbility` middleware (unauthenticated, ability-denied, policy-denied).

The helper is wrapped in try/catch so logging failures never affect the request
path. Successful operations log at `info`; suspicious/denied requests log at
`warning`. Operators wanting a failures-only feed can set `LOG_AUDIT_LEVEL=warning`.

Includes a feature test suite covering the helper, the webhook providers and the
new auth/authorization log paths.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

config/logging.php

@@ -132,6 +132,14 @@ return [
             'level' => 'warning',
             'days' => 14,
         ],
+
+        'audit' => [
+            'driver' => 'daily',
+            'path' => storage_path('logs/audit.log'),
+            'level' => env('LOG_AUDIT_LEVEL', 'info'),
+            'days' => env('LOG_AUDIT_DAYS', 90),
+            'replace_placeholders' => true,
+        ],
     ],
 
 ];