feat: replace token-based impersonation with enterprise-grade header-based system

Replaces the insecure token-in-localStorage approach with a header-based
impersonation system backed by cache sessions and MFA verification.

Key changes:
- New impersonation_sessions audit table (immutable, ULID PK)
- MFA verification required to start impersonation (TOTP/email/backup)
- X-Impersonate-User header + HandleImpersonation middleware
- Per-request auth context swap (admin session never modified)
- IP pinning, sensitive route blocking, no nesting, sliding 60-min TTL
- Activity log auto-tagged with impersonated_by during sessions
- Frontend: sessionStorage, BroadcastChannel sync, countdown timer
- ImpersonateDialog with reason + MFA verification flow
- 26 comprehensive tests covering core, middleware, audit, lifecycle

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Http/Requests/Admin/StartImpersonationRequest.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests\Admin;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class StartImpersonationRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true; // Authorization handled in service
+    }
+
+    /** @return array<string, mixed> */
+    public function rules(): array
+    {
+        return [
+            'reason' => ['required', 'string', 'min:5', 'max:500'],
+            'mfa_code' => ['required', 'string'],
+            'mfa_method' => ['required', 'string', 'in:totp,email,backup_code'],
+        ];
+    }
+}