`pub struct Billing`

Billing is a service which polls the database, creates invoices, and attempts to collect payment for invoices.

Members:

`pub fn new(repo: Repo, robot: Robot) -> Self`

Calls self.tick in a loop every hour.

Iterates over repo.list_activity since last run and does the following:

For any create_relay|update_relay|activate_relay activity if this is the first non-free relay for the tenant, update tenant's billing anchor to the time the relay was created.

Also iterates over repo.list_tenants() and for each tenant calls self.generate_invoice_if_due(tenant) and self.collect_outstanding(tenant).

Skip tenants that have a pending invoice or have no active non-free relays.
Compute current billing period from tenant.billing_anchor as rolling monthly windows: [period_start, period_end).
Only generate an invoice once the period has closed (now >= period_end).
Load activity needed to compute usage for the tenant using repo.list_activity.
Calculate how many hours (rounded up) each relay was active during the window per paid plan.
If total invoice amount is 0, return.
Generate a bolt11 invoice. If this fails, log the error and return.
Generate an invoice for the tenant and an invoice item for each relay.
Persist invoice + items atomically using repo.create_invoice.

Load pending tenant invoices and attempt to collect each one.
If attempted_at is less than 24 hours ago, skip it.
If the bolt11 invoice has been paid out of band, call repo.mark_invoice_paid and return.
If the tenant has a nwc_url, attempt to pay the invoice with nwc.
If collection succeeds, call repo.mark_invoice_paid.
If collection fails, populate repo.mark_invoice_attempted.
If nwc isn't set up or fails and sent_at is not set:
- Send a NIP 17 DM to the user with the invoice included.
- Call repo.mark_invoice_sent.
If the invoice is 7 days past created_at, call repo.mark_invoice_closed.