Add admin audit log viewer and system settings

Phase 5 (Admin Panel):
- Audit log viewer: searchable with action/date filters, expandable
  rows showing JSON changes, color-coded action chips, user avatars
- System settings: tabbed page (General, API Credentials, Billing,
  Notifications) with masked sensitive values, per-group save
- Settings model with get/set/getGroup/setGroup helpers
- Settings migration for key-value store with groups
- 52 tests passing, build clean

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

website/app/Models/Setting.php

@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+class Setting extends Model
+{
+    protected $fillable = [
+        'key',
+        'value',
+        'group',
+    ];
+
+    /**
+     * Get a setting value by key.
+     */
+    public static function get(string $key, mixed $default = null): mixed
+    {
+        $setting = static::query()->where('key', $key)->first();
+
+        return $setting?->value ?? $default;
+    }
+
+    /**
+     * Set a setting value by key.
+     */
+    public static function set(string $key, mixed $value, string $group = 'general'): void
+    {
+        static::query()->updateOrCreate(
+            ['key' => $key],
+            ['value' => $value, 'group' => $group],
+        );
+    }
+
+    /**
+     * Get all settings for a given group as a key-value array.
+     *
+     * @return array<string, string|null>
+     */
+    public static function getGroup(string $group): array
+    {
+        return static::query()
+            ->where('group', $group)
+            ->pluck('value', 'key')
+            ->toArray();
+    }
+
+    /**
+     * Set multiple settings at once for a given group.
+     *
+     * @param  array<string, string|null>  $settings
+     */
+    public static function setGroup(string $group, array $settings): void
+    {
+        foreach ($settings as $key => $value) {
+            static::set($key, $value, $group);
+        }
+    }
+}