Add company invitation system (backend)

New feature allowing company owners/admins to invite users by email with
a specific company-scoped role.

Database:
- New company_invitations table (company_id, email, role_id, token,
  status, invited_by, expires_at)

Backend:
- CompanyInvitation model with pending/forUser scopes
- InvitationService: invite, accept, decline, getPendingForUser
- CompanyInvitationMail with markdown email template
- InvitationController (company-scoped): list, send, cancel invitations
- InvitationResponseController (user-scoped): pending, accept, decline
- BootstrapController returns pending_invitations in response
- CompanyMiddleware handles zero-company users gracefully

Tests: 9 feature tests covering invite, accept, decline, cancel, expire,
duplicate prevention, and bootstrap integration.

database/migrations/2026_04_03_204854_create_company_invitations_table.php

@@ -0,0 +1,32 @@
+<?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::create('company_invitations', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('company_id')->constrained()->cascadeOnDelete();
+            $table->foreignId('user_id')->nullable()->constrained()->nullOnDelete();
+            $table->string('email');
+            $table->foreignId('role_id')->constrained('roles')->cascadeOnDelete();
+            $table->string('token')->unique();
+            $table->enum('status', ['pending', 'accepted', 'declined', 'expired'])->default('pending');
+            $table->foreignId('invited_by')->constrained('users')->cascadeOnDelete();
+            $table->timestamp('expires_at');
+            $table->timestamps();
+
+            $table->index(['email', 'status']);
+            $table->index(['user_id', 'status']);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('company_invitations');
+    }
+};