Update README/documentation to clarify how sessions work.

Overview

The current template documentation could be updated to provide clarity on how multi-tenant applications should handle resource ownership in Shopify apps.

Currently, there is a sessions table as well as a corresponding Session model, but no User model or users table which comes standard in fresh installations of Laravel. Documentation could be updated to address the following questions:

• Does Session replace User? Is it safe to create foreign key constraints against a session?
• How are the user_* columns in the sessions table populated? Am I supposed to populate those fields myself or is there underlying code that does so automatically?
• Are sessions unique per shop? Can a shop have multiple sessions?
• How do I get the current authenticated session/shop/user? Typically this used to be done with $request->user() or \Illuminate\Facades\Auth::user(), but using either results in an error.

Type

• New feature
• Changes to existing features

Motivation

What inspired this feature request? What problems were you facing?

I created a fresh application using this template but couldn't find any guidance on if sessions have a unique one-to-one with a shop/user or not.

Checklist

• I have described this feature request in a way that is actionable (if possible)

Start with the README/documentation, then inspect the sessions table and Session model referenced in the issue. Verify how user_* fields, session uniqueness, and current authenticated shop or user are handled, including the behavior of $request->user() and Auth::user(). Done means documenting clear answers to each listed question for fresh Shopify app installations.