[BACKEND] Add role and access control to users collection

[aleckshen]

Is your feature request related to a problem? Please describe

The Users collection has no way to distinguish between admins and regular users, meaning anyone with an account has the same level of access to data and the admin panel.

Describe the solution you'd like

Add a role field to the Users collection and implement access control across the board.

1. Create src/payload/access/isAdmin.ts — a reusable helper that returns true if the logged-in user has the admin role.

2. Add a role field to src/payload/collections/Users.ts:

Field	Type	Required	Notes
role	select	yes	Options: admin / user, defaults to user. Only admins can update this field.

3. Add collection-level access control to Users:

Operation	Rule
create	Anyone (open registration)
read	Admins can read all; users can only read their own profile
update	Admins can update all; users can only update their own profile
delete	Admins only

4. Lock the admin panel — update the admin config so the panel is hidden from non-admin users.

Describe alternatives you've considered

No alternatives considered — role-based access is a standard requirement for any multi-user system with an admin panel.

Additional context

Ref: https://payloadcms.com/docs/access-control/overview

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