feat(tariff): Tariff Lab — pure session-pricing simulator

Test rates "in time" (overnight windows, daily caps, overstay) in seconds against
any tariff version, instead of waiting hours/days. No real ledger writes.

- Extract priceSession() into @parking/shared: the grace/overstay wrapper over
  computeFee (unpaid -> entry..now; within-grace -> settled 0; grace-expired ->
  overstay, a fresh period from grace-expiry). PayStation.quote() now calls it so
  the booth and the lab can never diverge.
- API (tariffs.ts, tariff:read, read-only): POST /api/tariff/simulate prices a
  hypothetical session (active/any version/inline structure) and returns the
  priceSession outcome + a 30m..3d duration curve (see where the daily cap flattens);
  GET /api/tariff/simulate/session/:identity prefills from a real ledger session.
- UI TariffLab.tsx at Setup -> "Tariff Lab": version picker, entry/asOf times,
  optional payment+grace, category, and load-a-real-ticket. Admin-gated, available
  on-site (useful to quote a dispute).
- 4 new priceSession unit tests incl. the ticket-1245791632490 overstay-not-zero
  regression (40 pass). i18n lab.* + nav.tariffLab (sq+en). Verified live via the UI.

Wiki: tariff (priceSession + Tariff Lab as-built), log.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/server/src/pay-station.ts

@@ -1,5 +1,5 @@
 import { desc, eq, ledgerEvents, sessions, subscriptions, tariffVersions, tariffs, type Db } from "@parking/db";
-import { computeFee, type TariffStructure, type Tender } from "@parking/shared";
+import { priceSession, type TariffStructure, type Tender } from "@parking/shared";
 import type { FastifyBaseLogger } from "fastify";
 import type { EventLog } from "./event-log.js";
 
@@ -128,15 +128,6 @@ export class PayStation {
     const entry = this.#openEntry(identity);
     if (!entry) throw new NoOpenSessionError(identity);
 
-    // If the latest payment's walk-back grace has expired, this is an overstay: anchor
-    // the new billing period at grace-expiry (paidAt + graceExitMin). Otherwise price
-    // from entry (first payment, or a still-within-grace re-quote of the same stay).
-    const last = this.#lastPayment(identity);
-    const graceExpiryMs =
-      last && last.graceExitMin != null ? Date.parse(last.paidAt) + last.graceExitMin * 60_000 : null;
-    const overstay = graceExpiryMs != null && Date.now() > graceExpiryMs;
-    const periodStart = overstay ? new Date(graceExpiryMs!).toISOString() : entry.occurredAt;
-
     // The tariff in force is keyed to ENTRY (the version frozen for this session), even
     // for an overstay period — the customer keeps the rate card they entered under.
     const tv = this.#tariffVersionFor(entry.occurredAt);
@@ -147,13 +138,24 @@ export class PayStation {
     // both read it from there, so a V2 category tariff yields the same amount at the
     // booth and at exit. Absent (legacy/V1) ⇒ undefined ⇒ category-agnostic pricing.
     const category = (entry.payload as { category?: string } | null)?.category;
-    const amountMinor = computeFee(periodStart, new Date().toISOString(), structure, category);
+
+    // Pure pricing shared with the Tariff Lab (priceSession). Only the latest payment
+    // matters for grace/overstay; pass it through. Overstay → fresh period from
+    // grace-expiry; within-grace → settled; unpaid → entry→now running total.
+    const last = this.#lastPayment(identity);
+    const p = priceSession(
+      entry.occurredAt,
+      new Date().toISOString(),
+      structure,
+      last ? [last] : [],
+      category,
+    );
     return {
       identity,
       enteredAt: entry.occurredAt,
-      periodStart,
-      amountMinor,
-      overstay,
+      periodStart: p.periodStart,
+      amountMinor: p.amountMinor,
+      overstay: p.overstay,
       currency: tv.currency,
       tariffVersionId: tv.id,
       graceExitMin: structure.gracePeriodExitMin,