[BACKEND] Add Users collection

[aleckshen]

Is your feature request related to a problem? Please describe

The Users collection currently only stores an email address, which isn't enough to identify or manage members effectively.

Describe the solution you'd like

Field	Type	Required	Notes
firstName	text	yes
lastName	text	yes
upiOrEmail	text	no	Student ID e.g. ashe123
membershipStatus	select	yes	Options: member / non-member, defaults to non-member
phone	text	no

Also update auth: true to set a tokenExpiration of 43200 (12 hours) so users stay logged in longer. (This can be flexible to change later down the line)

Describe alternatives you've considered

No alternatives considered, these are straightforward data fields needed for member management.

Additional context

After making changes, pnpm payload generate:types should be run to regenerate payload-types.ts.

Ref: https://payloadcms.com/docs/fields/overview

BEFORE MERGING

• Code generation run (hint: pnpm typegen)
• PR Reviewed (For non-trivial changes)
• All required PR checks passing

[pav-dev98]

I'd like to work on this!