Assessment groups can now define billing frequency (monthly, quarterly,
annual) with configurable due months and due day. Invoice generation
respects each group's schedule - only generating invoices when the
selected month is a billing month for that group. Adds a generation
preview showing which groups will be billed, period tracking on
invoices, and billing period context in the payments UI.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
