Monthly Billing Cycle

M-1 + M-4: monthly invoice generation with weekend/holiday adjustment.

BILLING_RULES.md Section 3:

• Invoice date: 26th of M+1 calendar-basis
• Weekend/holiday adjustment: next business day

The Cloud Scheduler job runs DAILY (cron under our control). Each run:

1. Calls invoice_date_for_billing_cycle(today) to decide whether today is the canonical invoice date (M-4 logic encapsulated there).
2. If not the invoice day, no-ops and returns success.
3. If it IS the invoice day, calls InvoiceGenerator.generate_monthly_invoices(today).

Idempotent: the generator skips any attribution row already invoiced. Two firings on the same canonical day will not double-charge.