refactor(webhook): encrypt manual webhook secrets and tighten HMAC verification

- Auto-generate a 40-char random secret for each manual_webhook_secret_* column on Application creation so new apps are never left with an empty secret.
- Add encrypted cast for the four webhook-secret columns; backfill migration re-encrypts existing plaintext values and fills missing ones.
- Reject webhook deliveries when the stored secret is empty (GitHub, GitLab, Bitbucket, Gitea manual endpoints).
- Bitbucket: require the sha256 algorithm prefix on X-Hub-Signature instead of trusting the client-supplied algo.
- GitLab: drop the ?? '' fallback on the token comparison.

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

app/Http/Controllers/Webhook/Gitlab.php

@@ -100,7 +100,16 @@ class Gitlab extends Controller
             }
             foreach ($applications as $application) {
                 $webhook_secret = data_get($application, 'manual_webhook_secret_gitlab');
-                if (! hash_equals($webhook_secret ?? '', $x_gitlab_token ?? '')) {
+                if (empty($webhook_secret)) {
+                    $return_payloads->push([
+                        'application' => $application->name,
+                        'status' => 'failed',
+                        'message' => 'Webhook secret not configured.',
+                    ]);
+
+                    continue;
+                }
+                if (! hash_equals($webhook_secret, $x_gitlab_token ?? '')) {
                     $return_payloads->push([
                         'application' => $application->name,
                         'status' => 'failed',