Add ProductPrice.cost_amount (integer cents) — the cost to deliver one unit at
that price: manufacturing, shipping, a content license or royalty; per billing
period for recurring prices. Shop now reports margin natively: subscriptionMetrics()
returns `cogs` + `net_mrr`, and Shop::netMrr() gives MRR after cost of goods.
Additive: cost_amount defaults to 0, so existing consumers are unchanged.
Covered by MrrMetricsTest; full suite green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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>
Fabian @ Blax Software