#52 [Admin Panel] Invitation System - Send Invitation

Implement POST /api/v1/admin/invitations endpoint to invite users to tenant. **API Specification:** - Endpoint: POST /api/v1/admin/invitations - Permission Required: manage_users - Request Body: {email, roles[]?, message?, expires_in_days?} - Response: {invitation_id, email, roles[], expires_at, invitation_url} **Requirements:** - Generate secure invitation token (32-byte random) - Send invitation email with personalized link - Configurable expiration (default: 7 days) - Pre-assign roles (applied on acceptance) - Optional custom message in email - Prevent duplicate invitations to same email - Log audit event **Database Schema (invitations table):** - invitation_id (UUID), tenant_id - email, roles[] (JSONB) - token_hash (SHA256) - message, expires_at - created_at, created_by - accepted_at, accepted_by_user_id - status (pending, accepted, expired, cancelled) **Email Template:** - Include tenant name, inviter name - Custom message if provided - Expiration warning - Accept invitation button/link **IMPORTANT:** Implement with ultra attention to details and correctness. **BEFORE CLOSING:** Take screenshot showing invitation creation. Add comment summarizing implementation.