Add CSV export endpoint for UAIC weekly workflow

[evanautianle]

User story:
As a UAIC executive
I want to export membership/payment records as CSV
So that I can run the same weekly operational workflow currently done in Squarespace

Acceptance Criteria (AC):
A backend endpoint returns membership/payment data as downloadable CSV.
CSV includes agreed weekly workflow fields (minimum: name, email, student ID, membership status, payment date).
Endpoint supports weekly filtering (e.g., paid within date range or last 7 days).
CSV headers are stable and documented for UAIC operations.
Export works with records created by the new Stripe payment flow.
Endpoint handles empty datasets by returning a valid CSV with headers only.

Definition of done:
AC met
PR reviewed
merged to main/feature branch