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/database/factories/ImpersonationSessionFactory.php

@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Database\Factories;
+
+use App\Models\ImpersonationSession;
+use App\Models\User;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+/** @extends Factory<ImpersonationSession> */
+final class ImpersonationSessionFactory extends Factory
+{
+    protected $model = ImpersonationSession::class;
+
+    /** @return array<string, mixed> */
+    public function definition(): array
+    {
+        return [
+            'admin_id' => User::factory(),
+            'target_user_id' => User::factory(),
+            'reason' => fake()->sentence(),
+            'mfa_method' => 'totp',
+            'ip_address' => fake()->ipv4(),
+            'user_agent' => fake()->userAgent(),
+            'started_at' => now(),
+            'expires_at' => now()->addMinutes(60),
+            'actions_count' => 0,
+        ];
+    }
+
+    public function ended(string $reason = 'manual'): static
+    {
+        return $this->state(fn () => [
+            'ended_at' => now(),
+            'end_reason' => $reason,
+        ]);
+    }
+
+    public function expired(): static
+    {
+        return $this->state(fn () => [
+            'started_at' => now()->subHours(2),
+            'expires_at' => now()->subHour(),
+            'ended_at' => now()->subHour(),
+            'end_reason' => 'expired',
+        ]);
+    }
+}