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>

database/migrations/2026_04_19_000000_backfill_and_encrypt_webhook_secrets.php

@@ -0,0 +1,59 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\Crypt;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+use Illuminate\Support\Str;
+
+class BackfillAndEncryptWebhookSecrets extends Migration
+{
+    public function up(): void
+    {
+        $columns = [
+            'manual_webhook_secret_github',
+            'manual_webhook_secret_gitlab',
+            'manual_webhook_secret_bitbucket',
+            'manual_webhook_secret_gitea',
+        ];
+
+        Schema::table('applications', function ($table) use ($columns) {
+            foreach ($columns as $col) {
+                $table->text($col)->nullable()->change();
+            }
+        });
+
+        try {
+            DB::table('applications')->chunkById(100, function ($apps) use ($columns) {
+                foreach ($apps as $app) {
+                    $updates = [];
+                    foreach ($columns as $col) {
+                        $current = $app->{$col};
+
+                        if (empty($current)) {
+                            $updates[$col] = Crypt::encryptString(Str::random(40));
+
+                            continue;
+                        }
+
+                        try {
+                            Crypt::decryptString($current);
+
+                            continue;
+                        } catch (Exception) {
+                            // Not encrypted yet
+                        }
+
+                        $updates[$col] = Crypt::encryptString($current);
+                    }
+                    if ($updates !== []) {
+                        DB::table('applications')->where('id', $app->id)->update($updates);
+                    }
+                }
+            });
+        } catch (Exception $e) {
+            echo 'Backfilling and encrypting webhook secrets failed.';
+            echo $e->getMessage();
+        }
+    }
+}