feat(billing): B-2.7 Stripe Checkout 3 plans CAD/TVQ + Apple/Google Pay

Adds the customer-facing checkout flow under /checkout/<plan>:
- src/billing/plans.py — Plan dataclass + 3 plans (DictIA 8 / 16 / Cloud),
  monthly + yearly Price IDs resolved from STRIPE_DICTIA_*_{SETUP,MONTHLY,YEARLY} env.
- src/billing/stripe_client.py — lazy stripe.api_key init, get_or_create_customer
  (persists user.stripe_customer_id), create_checkout_session with mode=subscription,
  currency=cad, automatic_tax=true (TPS 5% + TVQ 9.975%), billing_address_collection,
  metadata on both Session and Subscription for the B-2.8 webhook.
- src/billing/routes.py — GET /checkout/<plan>?period=monthly|yearly returns 303
  redirect to Stripe-hosted Checkout. Friendly French flash + redirect to /tarifs
  on unknown plan, missing STRIPE_SECRET_KEY, missing Price IDs, or Stripe API error.
  GET /checkout/success and /checkout/cancel render brand-tokenized templates that
  extend marketing/base.html.
- templates/billing/{success,cancel}.html — explicit "activé sous quelques minutes"
  note (webhook is async), aucun montant prélevé reassurance on cancel.
- config/env.stripe.example — env vars + Stripe Dashboard setup checklist
  (CAD activation, Stripe Tax registrations, Apple/Google Pay enable, webhook).
- tests/test_stripe_checkout.py — 25 tests covering plans, stripe_client, routes,
  and the _PUBLIC_INDEXABLE_ENDPOINTS integration. Stripe SDK mocked via
  unittest.mock.patch (no network). Windows manual driver included.

Webhook (B-2.8) will be the source of truth for user.subscription_status.
This task only mutates user.stripe_customer_id (identity, not state).
Existing pricing CTAs in templates/marketing/_partials/_pricing_tiers.html
already link to /checkout/<slug> (verified) — no marketing template touched.

Tests: 25/25 new + 89/89 prior pass on Windows manual driver.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

templates/billing/success.html

@@ -0,0 +1,87 @@
+{% extends 'marketing/base.html' %}
+
+{% block title %}{{ title or 'Paiement confirmé — DictIA' }}{% endblock %}
+{% block description %}Paiement confirmé. Votre abonnement DictIA sera activé sous quelques minutes. Vous recevrez un courriel de confirmation.{% endblock %}
+
+{% block content %}
+
+{# ===== HERO ===== #}
+<section class="bg-brand-navy text-white py-20" aria-labelledby="page-title">
+  <div class="max-w-[820px] mx-auto px-6 text-center">
+    <div class="w-20 h-20 grad-bg rounded-full mx-auto mb-6 flex items-center justify-center text-4xl shadow-cta" aria-hidden="true">✓</div>
+    <p class="eyebrow grad-text mb-4">PAIEMENT CONFIRMÉ</p>
+    <h1 id="page-title" class="text-[clamp(2.25rem,4vw,3.5rem)] font-black mb-4">
+      Merci&nbsp;! Votre <span class="grad-text">paiement est confirmé</span>.
+    </h1>
+    <p class="text-lg text-white/80">
+      Votre abonnement sera activé sous quelques minutes. Vous recevrez un courriel de confirmation à l'adresse associée à votre compte.
+    </p>
+  </div>
+</section>
+
+{# ===== NEXT STEPS ===== #}
+<section class="bg-brand-bg py-20" aria-labelledby="next-steps-title">
+  <div class="max-w-[820px] mx-auto px-6">
+    <h2 id="next-steps-title" class="text-[clamp(1.75rem,2.5vw,2.25rem)] font-black mb-8 text-brand-navy text-center">
+      Prochaines étapes.
+    </h2>
+
+    <ol class="space-y-6">
+      <li class="bg-white p-6 rounded-[18px] border border-brand-border flex gap-4">
+        <span class="grad-bg text-white font-black w-10 h-10 rounded-full flex items-center justify-center flex-shrink-0 shadow-cta" aria-hidden="true">1</span>
+        <div>
+          <h3 class="font-bold text-brand-navy mb-1">Confirmation par courriel</h3>
+          <p class="text-sm text-brand-navy/80 leading-relaxed">
+            Vous recevrez un reçu détaillé (avec TPS et TVQ ventilées) dans les prochaines minutes. Vérifiez vos pourriels si rien n'arrive après 10&nbsp;minutes.
+          </p>
+        </div>
+      </li>
+
+      <li class="bg-white p-6 rounded-[18px] border border-brand-border flex gap-4">
+        <span class="grad-bg text-white font-black w-10 h-10 rounded-full flex items-center justify-center flex-shrink-0 shadow-cta" aria-hidden="true">2</span>
+        <div>
+          <h3 class="font-bold text-brand-navy mb-1">Activation de votre abonnement</h3>
+          <p class="text-sm text-brand-navy/80 leading-relaxed">
+            Votre statut d'abonnement sera mis à jour automatiquement dès que Stripe confirme la transaction (généralement sous 2&nbsp;minutes). Aucune action requise de votre part.
+          </p>
+        </div>
+      </li>
+
+      <li class="bg-white p-6 rounded-[18px] border border-brand-border flex gap-4">
+        <span class="grad-bg text-white font-black w-10 h-10 rounded-full flex items-center justify-center flex-shrink-0 shadow-cta" aria-hidden="true">3</span>
+        <div>
+          <h3 class="font-bold text-brand-navy mb-1">Mise en service</h3>
+          <p class="text-sm text-brand-navy/80 leading-relaxed">
+            Pour les forfaits <strong>DictIA&nbsp;Cloud</strong>&nbsp;: accès immédiat depuis votre tableau de bord.<br>
+            Pour les forfaits <strong>DictIA&nbsp;8</strong> et <strong>DictIA&nbsp;16</strong> (on-premise)&nbsp;: notre équipe vous contactera sous 1&nbsp;jour ouvrable pour planifier l'installation (~2&nbsp;semaines).
+          </p>
+        </div>
+      </li>
+    </ol>
+
+    {% if session_id %}
+    <p class="text-xs text-brand-navy/60 mt-8 text-center font-mono break-all">
+      Référence&nbsp;: {{ session_id }}
+    </p>
+    {% endif %}
+  </div>
+</section>
+
+{# ===== CTA ===== #}
+<section class="bg-white py-16" aria-labelledby="cta-title">
+  <div class="max-w-[820px] mx-auto px-6 text-center">
+    <h2 id="cta-title" class="text-[clamp(1.5rem,2vw,2rem)] font-black mb-4 text-brand-navy">
+      Une question&nbsp;?
+    </h2>
+    <p class="text-base text-brand-navy/80 mb-6">
+      Notre équipe est joignable à <a href="mailto:info@dictia.ca" class="grad-text font-semibold hover:underline">info@dictia.ca</a> ou au <a href="tel:+15819968471" class="grad-text font-semibold hover:underline">(581)&nbsp;996-8471</a>.
+    </p>
+    <div class="flex flex-col sm:flex-row gap-4 justify-center">
+      {% from 'macros/button.html' import button %}
+      {{ button('Retour à l\'accueil', href='/', variant='ghost', size='lg') }}
+      {{ button('Voir les tarifs', href='/tarifs', variant='secondary', size='lg') }}
+    </div>
+  </div>
+</section>
+
+{% endblock %}