feat(billing): add billing manager for the current billing period

[RapidPoseidon]

Adds a billing module to the SDK with a RapidataBillingManager, reachable as client.billing.

period = client.billing.get_current_billing_period()

period.start_date, period.end_date   # the period's window
period.status                        # "Open" while it is still accruing
period.outstanding_cost              # net cost accrued so far, in USD
period.credits                       # remaining prepaid credit, or None

What it returns

BillingPeriod is a frozen dataclass with id, start_date, end_date, status, outstanding_cost, gross_cost, discount, response_count and credits.

Where the numbers come from

Two payment-service endpoints, folded into one object:

• GET /billing-period/active — the period, its dates/status, and its costs. outstanding_cost is the net cost (gross minus discounts), i.e. what the period would be invoiced for today.
• GET /billing/summary — availableAmount, the remaining prepaid balance. Credits sit on the organization's billing account rather than on the period, which is why they need the second call.

availableAmount is only populated when the organization is on a prepaid plan (CreditLine == 0), so credits is None on a usage-billed plan rather than a misleading 0.

Both endpoints are IsAuthenticated-only for the caller's own organization, so no extra role is needed.

Notes

• A billing period only opens once there is something to bill, so get_current_billing_period() raises the usual RapidataError (404, "No active billing period found") for an organization that has never run a billable job. Documented in the guide.
• Adds a PaymentService to the service layer (matching the existing per-backend-service grouping) — no manager reached the payment service before.
• New guide at docs/billing.md, linked from the nav and the llms.txt sections.
• 4 pre-existing test failures under tests/rapidata_client/audience/ are unrelated — they fail on main too.

🔗 Session: https://poseidon.rapidata.internal/chat/session-93f851b0