Submit invitation request to join organization

Submit a request to join a specific organization.

Process:

1. Validates user information and organization
2. Checks for existing invitations (auto-approves if found)
3. Creates invitation request record
4. Sends notifications to organization admins
5. Sends confirmation to requesting user

Auto-Approval: If the user already has a valid invitation from the organization, the request is automatically approved and user account created.

Request Body:

{
    "email": "user@example.com",
    "first_name": "John",
    "last_name": "Doe",
    "mobile": "+1234567890",
    "requested_to": 123,
    "organization": 456,
    "password": "securepassword"
}

Returns: Invitation request details or auto-approval confirmation