Add deployment queue limit to prevent queue bombing

- Add configurable deployment_queue_limit server setting (default: 25)
- Check queue size before accepting new deployments
- Return 429 status for webhooks/API when queue is full (allows retry)
- Show error toast in UI when queue limit reached
- Add UI control in Server Advanced settings

Fixes #6708

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

database/migrations/2025_12_04_134435_add_deployment_queue_limit_to_server_settings.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('server_settings', function (Blueprint $table) {
+            $table->integer('deployment_queue_limit')->default(25)->after('concurrent_builds');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('server_settings', function (Blueprint $table) {
+            $table->dropColumn('deployment_queue_limit');
+        });
+    }
+};