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/lib/changelog.ts

@@ -1,5 +1,5 @@
 // Mirrors CHANGELOG.md at the repo root — update both together.
-export const APP_VERSION = "0.72.0";
+export const APP_VERSION = "0.73.0";
 
 export type ChangelogEntry = {
   version: string;
@@ -11,6 +11,15 @@ export type ChangelogEntry = {
 };
 
 export const CHANGELOG: ChangelogEntry[] = [
+  {
+    version: "0.73.0",
+    date: "2026-07-23 14:00",
+    added: [
+      "Real Stripe billing for Pro/Family: subscribe and manage your plan yourself at Settings -> Billing (Checkout + Stripe's hosted Customer Portal for cancel/upgrade/card updates), instead of only an admin being able to change your tier. Cancel takes effect at the end of the paid period.",
+      "Admin -> Billing: connection status, subscriber counts, past-due payments needing attention, recent billing events. Admin -> Tier Limits gained Stripe Product/Price ID fields per tier.",
+    ],
+    notes: "Family tier is purchasable solo but doesn't yet support inviting other accounts into a shared subscription — that's a separate, larger piece tracked in plans/STRIPE_PLAN.md, deliberately shipped after solo billing.",
+  },
   {
     version: "0.72.0",
     date: "2026-07-23 09:30",