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>

app/Http/Middleware/ApiAbility.php

@@ -2,6 +2,7 @@
 
 namespace App\Http\Middleware;
 
+use Illuminate\Auth\AuthenticationException;
 use Laravel\Sanctum\Http\Middleware\CheckForAnyAbility;
 
 class ApiAbility extends CheckForAnyAbility
@@ -14,11 +15,22 @@ class ApiAbility extends CheckForAnyAbility
             }
 
             return parent::handle($request, $next, ...$abilities);
-        } catch (\Illuminate\Auth\AuthenticationException $e) {
+        } catch (AuthenticationException $e) {
+            auditLog('api.auth.unauthenticated', [
+                'reason' => $e->getMessage(),
+                'required_abilities' => $abilities,
+            ], 'warning');
+
             return response()->json([
                 'message' => 'Unauthenticated.',
             ], 401);
         } catch (\Exception $e) {
+            auditLog('api.auth.ability_denied', [
+                'required_abilities' => $abilities,
+                'token_id' => $request->user()?->currentAccessToken()?->id,
+                'reason' => $e->getMessage(),
+            ], 'warning');
+
             return response()->json([
                 'message' => 'Missing required permissions: '.implode(', ', $abilities),
             ], 403);