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/Http/Resources/CompanyInvitationResource.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace App\Http\Resources;
+
+use Illuminate\Http\Request;
+use Illuminate\Http\Resources\Json\JsonResource;
+
+class CompanyInvitationResource extends JsonResource
+{
+    public function toArray(Request $request): array
+    {
+        return [
+            'id' => $this->id,
+            'company_id' => $this->company_id,
+            'email' => $this->email,
+            'token' => $this->token,
+            'status' => $this->status,
+            'expires_at' => $this->expires_at,
+            'created_at' => $this->created_at,
+            'company' => $this->when($this->relationLoaded('company'), function () {
+                return new CompanyResource($this->company);
+            }),
+            'role' => $this->when($this->relationLoaded('role'), function () {
+                return new RoleResource($this->role);
+            }),
+            'invited_by' => $this->when($this->relationLoaded('invitedBy'), function () {
+                return new UserResource($this->invitedBy);
+            }),
+        ];
+    }
+}