fix(image-scan): let a server set the scanner memory limit

Grype was killed at the fixed 1 GiB container limit on large images.

- Add the per-server setting image_scan_memory_mb (1, 2, 4 or 8 GB,
  default 1 GB) on the Vulnerabilities page and in the
  /servers/{uuid}/image-scanning API.
- Set GOMEMLIMIT to 85% of the limit, so Go collects garbage before
  the container limit kills the scanner.
- The out-of-memory error now tells the user to raise the limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

database/migrations/2026_11_09_130000_add_image_scan_memory_mb_to_server_settings_table.php

@@ -0,0 +1,22 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('server_settings', function (Blueprint $table) {
+            $table->unsignedInteger('image_scan_memory_mb')->default(1024);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('server_settings', function (Blueprint $table) {
+            $table->dropColumn('image_scan_memory_mb');
+        });
+    }
+};