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/migrations/2026_04_16_100000_create_impersonation_sessions_table.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+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::create('impersonation_sessions', function (Blueprint $table) {
+            $table->ulid('id')->primary();
+            $table->foreignUlid('admin_id')->constrained('users')->cascadeOnDelete();
+            $table->foreignUlid('target_user_id')->constrained('users')->cascadeOnDelete();
+            $table->string('reason');
+            $table->string('mfa_method', 20);
+            $table->string('ip_address', 45);
+            $table->text('user_agent')->nullable();
+            $table->timestamp('started_at');
+            $table->timestamp('ended_at')->nullable();
+            $table->timestamp('expires_at');
+            $table->string('end_reason', 50)->nullable();
+            $table->unsignedInteger('actions_count')->default(0);
+
+            $table->index(['admin_id', 'ended_at']);
+            $table->index(['target_user_id', 'ended_at']);
+            $table->index('started_at');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('impersonation_sessions');
+    }
+};