Billing: complimentary and covered-amount enrollments, late usage corrections, launch-readiness fixes #139

Shipped on main and live (Stripe live since 2026-10-15 00:19 UTC):

• ed6a77aa: complimentary enrollments from sudo, with an allowance and a fee, and a choice past the allowance (stop, or charge the card); every workspace except flagon-io moved to Free
• f2c1aa0b: plan line name on checkout and receipts; the plan's card shows the usage month and the renewal date separately
• 1b4e6377: events replay and saved-card sync no longer bind undefined into D1
• 6790d3eb: billing docs (live payments, usage month and renewal, git operations, Actions concurrency)

These commits said "(#47)" with no closing keyword, so nothing closed the issue. Closing it by hand.