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>

src/billing/routes.py

@@ -0,0 +1,126 @@
+"""Billing routes — Stripe Checkout (B-2.7).
+
+URL space (prefix `/checkout`, set on billing_bp):
+- GET  /checkout/<plan>?period=monthly|yearly  → 303 redirect to Stripe-hosted Checkout
+- GET  /checkout/success?session_id=...        → confirmation page (async activation note)
+- GET  /checkout/cancel                        → friendly "no charge made" page
+
+The webhook route (B-2.8) is registered separately at /webhooks/stripe outside
+the /checkout prefix and is CSRF-exempt.
+"""
+import logging
+
+from flask import (
+    Blueprint, current_app, flash, redirect, render_template,
+    request, url_for,
+)
+from flask_login import current_user, login_required
+
+from src.billing import billing_bp
+from src.billing.plans import VALID_PERIODS, get_plan
+from src.billing.stripe_client import (
+    StripeNotConfiguredError,
+    create_checkout_session,
+    is_stripe_configured,
+)
+
+logger = logging.getLogger(__name__)
+
+
+@billing_bp.route('/<plan>')
+@login_required
+def checkout(plan):
+    """Initiate Stripe Checkout for the given plan + period.
+
+    Redirects to /tarifs with a French flash on any error (unknown plan,
+    Stripe not configured, plan Price IDs missing, Stripe API failure).
+    Returns a 303 See Other redirect to the Stripe-hosted Checkout on success
+    (303 is what Stripe documents for HTTP redirects to checkout.stripe.com).
+    """
+    plan_obj = get_plan(plan)
+    if plan_obj is None:
+        flash('Forfait inconnu.', 'danger')
+        return redirect(url_for('marketing.tarifs'))
+
+    period = request.args.get('period', 'monthly')
+    if period not in VALID_PERIODS:
+        period = 'monthly'
+
+    if not is_stripe_configured():
+        flash(
+            "Le paiement en ligne n'est pas disponible pour le moment. "
+            "Contactez info@dictia.ca pour finaliser votre abonnement.",
+            'warning',
+        )
+        return redirect(url_for('marketing.tarifs'))
+
+    if not plan_obj.is_configured():
+        flash(
+            "Ce forfait n'est pas encore configuré. Contactez info@dictia.ca.",
+            'warning',
+        )
+        return redirect(url_for('marketing.tarifs'))
+
+    success_url = url_for('billing.success', _external=True)
+    cancel_url = url_for('billing.cancel', _external=True)
+
+    try:
+        session = create_checkout_session(
+            plan_slug=plan,
+            period=period,
+            user=current_user,
+            success_url=success_url,
+            cancel_url=cancel_url,
+        )
+    except StripeNotConfiguredError as e:
+        logger.error('Stripe not configured at checkout: %s', e)
+        flash(
+            "Le paiement en ligne n'est pas disponible. "
+            "Contactez info@dictia.ca.",
+            'warning',
+        )
+        return redirect(url_for('marketing.tarifs'))
+    except ValueError as e:
+        logger.warning('Invalid checkout request: %s', e)
+        flash('Demande de paiement invalide.', 'danger')
+        return redirect(url_for('marketing.tarifs'))
+    except Exception as e:  # noqa: BLE001
+        logger.exception(
+            'Stripe Checkout creation failed for user %s plan %s: %s',
+            getattr(current_user, 'id', '?'), plan, e,
+        )
+        flash(
+            "Une erreur est survenue lors de l'ouverture du paiement. "
+            "Réessayez ou contactez info@dictia.ca.",
+            'danger',
+        )
+        return redirect(url_for('marketing.tarifs'))
+
+    # Stripe documents 303 See Other for hosted-Checkout redirects.
+    return redirect(session.url, code=303)
+
+
+@billing_bp.route('/success')
+def success():
+    """Post-payment confirmation page.
+
+    The session_id query param is preserved for optional client-side analytics
+    but is NOT trusted server-side — Stripe's webhook (B-2.8) is the source of
+    truth for subscription state. This page makes that asynchrony explicit
+    ("Votre abonnement sera activé sous quelques minutes.").
+    """
+    session_id = request.args.get('session_id')
+    return render_template(
+        'billing/success.html',
+        title='Paiement confirmé — DictIA',
+        session_id=session_id,
+    )
+
+
+@billing_bp.route('/cancel')
+def cancel():
+    """User cancelled the Stripe Checkout. No state to revert; no charge made."""
+    return render_template(
+        'billing/cancel.html',
+        title='Paiement annulé — DictIA',
+    )