Stripe Meters: €19 base + €8/1k MTU overage on the org customer

[axelgar]

What to build

Make Pro actually charge for usage above the included MTU. The Pro subscription becomes a €19 base price plus a metered price for MTU overage, billed to the org's Stripe customer (org = Stripe customer comes from PRD-A, #12).

• Create the test + live Stripe meter and metered price for €8 per 1,000 MTU over the included 10,000 (only STRIPE_PRICE_PRO exists today; the new price/meter ids go in env alongside it).
• Report overage as max(0, liveMTU − 10_000) / 1_000 × €8 worth of meter events.
• Reconcile from the counter's authoritative period MTU, not from incremental events, so a missed report self-heals. Recommended cadence: daily reconcile (confirm against Stripe Meters' aggregation — see PRD §8 open question).
• Attribute events to the org's Stripe customer.
• Test usage (the test-cap set from Test-mode flat-100 MTU cap (separate set, never bills) #4) must never reach the meter.

Acceptance criteria

• A Pro test-mode subscription crossing 10,000 MTU bills €19 base + €8/1k overage.
• Overage is computed from the authoritative counter MTU (reconcile survives a missed report).
• Meter events are keyed to the org's Stripe customer.
• Test-key usage never appears on the meter / invoice.
• New meter/price ids exist for both test and live.

Blocked by

• plans.ts: per-tier pricing/ceiling numbers + planFor #1 (needs mtuIncluded / mtuOveragePer1k)
• Stripe customer = org (lazy create on first checkout, id on org) #12 (PRD-A: Stripe customer = org)