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feat(billing): in-app embedded Stripe checkout + webhook hardening

Checkout previously used hosted ui_mode → window.location to checkout.stripe.com,
which pops out of the installed PWA into the system browser. Switch to embedded:

- API: ui_mode embedded_page (stripe-node v22 / API 2025-10 renamed the enum),
  return_url instead of success/cancel_url, returns client_secret.
- web: @stripe/react-stripe-js EmbeddedCheckout mounted in an in-app modal;
  NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY baked at build (Dockerfile arg + compose arg).
- .env.production.example: full Stripe section (was missing) + admin-email
  placeholder (INF-001).

Also bundled (same files): BILL-002 invoice.paid resets quota only on
subscription_cycle; BILL-003 webhook dedup rolled back on handler failure;
BILL-001 change-plan writes plan locally; BILL-004 webhook cross-checks
sub.customer before trusting metadata.orgId; INF-003 API routed off the raw
docker.sock through a locked-down tecnativa/docker-socket-proxy (CONTAINERS+POST).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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apps/web/Dockerfile

@@ -24,6 +24,11 @@ RUN pnpm install --frozen-lockfile
 FROM deps AS build
 ARG NEXT_PUBLIC_API_URL=http://localhost:4000
 ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL
+# Stripe publishable key — inlined into the client bundle so the embedded
+# checkout can initialise. Safe to expose (publishable, not secret). Empty
+# build = embedded checkout shows a "not configured" message until set.
+ARG NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=
+ENV NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=$NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY
 ENV NEXT_TELEMETRY_DISABLED=1
 COPY . .
 RUN pnpm --filter @bmm/web build
@@ -34,4 +39,9 @@ ENV NODE_ENV=production
 ENV NEXT_TELEMETRY_DISABLED=1
 WORKDIR /app/apps/web
 EXPOSE 3001
+# NOTE (INF-003): non-root `USER node` was reverted — `pnpm start` via corepack
+# can't reach its root-owned cache as the node user and the deploy health-check
+# doesn't cover web, so a broken web would deploy "green" but take the site down.
+# Re-enable only after switching the runtime CMD to invoke next directly
+# (node_modules/.bin/next) and smoke-testing the image locally.
 CMD ["pnpm", "start"]