Get current usage against effective limits

GET /billing/usage/ -- current usage against effective limits, per resource, plus billing_state, the plan snapshot, the current billing period's bounds, the plan/add-on split of each ceiling, per-scope attribution across the caller's pooled subtree, and the overage accrued so far this cycle. Resolved at the billing root, same as every other read in this app.

The "pull" half of "an scope can see where it stands". It reads usage through EntitlementService.effective_limit_from_resolved / usage_breakdown_for_root -- pre-resolved entry points onto the identical get_effective_limit / get_current_usage implementation check_limit / check_postpaid_allowance count against, and that vinta_billing.services.usage_warning_service.UsageWarningService (the "push" half -- proactive approaching-limit notifications) also reads its ceiling from -- so this endpoint, the enforcement checks, and the beat warning can never disagree about a number.

No permission beyond IsAuthenticated, deliberately -- a read never blocks, including for a RESTRICTED scope (a RESTRICTED scope has its writes blocked and sync paused, never its reads; an scope must be able to see exactly what it needs to resolve before it can act on it).