feat: real Stripe billing -- Checkout, Customer Portal, admin billing dashboard (v0.73.0)

Implements plans/STRIPE_PLAN.md sections 1-9 for solo Pro/Family
billing (family multi-user sharing, section 1a, deliberately deferred
-- flagged in that plan as the most novel/error-prone piece).

Decisions locked in: cancel/downgrade at period end (Stripe Portal
default), no trial period.

- lib/stripe.ts: single client factory reading STRIPE_SECRET_KEY via
  site-settings (DB overrides env, same pattern as every other
  provider key in this codebase).
- Webhook route rewritten on the real `stripe` SDK
  (stripe.webhooks.constructEvent replaces the hand-rolled HMAC
  verifier) and now handles the full event set: checkout.session.completed,
  customer.subscription.{updated,deleted}, invoice.{payment_failed,paid}.
  past_due deliberately never downgrades tier on its own -- Stripe
  retries the card first, recovering via invoice.paid or eventually
  giving up via subscription.deleted. Every handler audit-logs under
  billing.<event>. Dedup via the existing processed_stripe_events
  table, unchanged.
- Schema: tierDefinitions gained stripe{ProductId,PriceIdMonthly,
  PriceIdYearly} (the lookup table mapping a Price back to a tier on
  checkout); users gained stripeSubscriptionId/subscriptionStatus/
  currentPeriodEnd.
- New POST /api/v1/billing/checkout (creates a subscription Checkout
  Session, allow_promotion_codes: true), POST /api/v1/billing/portal
  (Stripe's hosted self-serve cancel/upgrade/card-update), GET
  /api/v1/billing/status.
- /settings/billing: current plan + renewal date, past_due warning,
  usage-vs-limits (reuses the existing UsageQuotaSection), plan
  comparison cards with per-tier Checkout buttons, manage-billing
  button once a Stripe customer exists.
- /admin/billing: connection status (test/live mode detection),
  subscriber counts, past-due list, recent billing audit events, link
  to Stripe Dashboard. Tier Limits page extended with the three Stripe
  price fields per tier (own render branch, not the numeric+Unlimited-
  switch machinery the existing fields use).

Before going live: an admin needs to create real Products/Prices in
Stripe, enter the IDs on Tier Limits, and configure the Stripe-side
webhook endpoint -- all operational steps the plan always called for,
none of it code.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/components/settings/manage-billing-button.tsx

@@ -0,0 +1,28 @@
+"use client";
+
+import { useState } from "react";
+import { toast } from "sonner";
+import { Button } from "@/components/ui/button";
+
+export function ManageBillingButton() {
+  const [loading, setLoading] = useState(false);
+
+  async function openPortal() {
+    setLoading(true);
+    try {
+      const res = await fetch("/api/v1/billing/portal", { method: "POST" });
+      if (!res.ok) throw new Error();
+      const { url } = (await res.json()) as { url: string };
+      window.location.assign(url);
+    } catch {
+      toast.error("Couldn't open the billing portal");
+      setLoading(false);
+    }
+  }
+
+  return (
+    <Button variant="outline" disabled={loading} onClick={() => { void openPortal(); }}>
+      {loading ? "Opening…" : "Manage billing"}
+    </Button>
+  );
+}