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.

app/Mail/CompanyInvitationMail.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace App\Mail;
+
+use App\Models\CompanyInvitation;
+use Illuminate\Bus\Queueable;
+use Illuminate\Mail\Mailable;
+use Illuminate\Mail\Mailables\Content;
+use Illuminate\Mail\Mailables\Envelope;
+use Illuminate\Queue\SerializesModels;
+
+class CompanyInvitationMail extends Mailable
+{
+    use Queueable, SerializesModels;
+
+    public function __construct(
+        public readonly CompanyInvitation $invitation,
+    ) {}
+
+    public function envelope(): Envelope
+    {
+        return new Envelope(
+            subject: __('You\'ve been invited to join :company', [
+                'company' => $this->invitation->company->name,
+            ]),
+        );
+    }
+
+    public function content(): Content
+    {
+        return new Content(
+            markdown: 'emails.company-invitation',
+            with: [
+                'invitation' => $this->invitation,
+                'companyName' => $this->invitation->company->name,
+                'roleName' => $this->invitation->role->title,
+                'inviterName' => $this->invitation->invitedBy->name,
+                'acceptUrl' => url("/invitations/{$this->invitation->token}/accept"),
+                'declineUrl' => url("/invitations/{$this->invitation->token}/decline"),
+            ],
+        );
+    }
+}