New billing rule: everyone renews on the 1st, full program price.
New enrollees pay pro-rata for remainder of first month, then renew
normally on the 1st going forward.

- advanceEnrollmentBillingDate() always sets next_billing_date to 1st
  of next month
- calculateFirstBillingDate() same: 1st of next month
- DB: set 44 un-renewed enrollments to 2026-08-01 (left 207 already-
  renewed at 2026-09-01 untouched)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>