Add Shop::mrr(), activeSubscribers(), mrrByProduct(), subscriptionMetrics()
and subscriberStatuses() to ShopService. Counts active/past_due/trialing
subscribers (not `ends_at IS NULL`, which silently drops still-billing
cancel-at-period-end subs and under-reports MRR), normalizes every interval
to a monthly run-rate (year/12, quarter/3, week & day by the average
days-per-month constant so they match Stripe), and returns `unpriced_items`
so a stale or incomplete price mirror is surfaced, never a silent undercount.
Additive only: no existing method changed. Covered by MrrMetricsTest.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
