feat: Stripe integration complete - products created, DB migration, payment_failed handler, credit buttons wired

- Create Stripe products/prices (Starter/Pro/Business monthly+yearly)
- Fix CRITICAL bug: add subscription_ends_at + cancel_at_period_end columns to users table
- Alembic migration: f6a7b8c9d0e1_add_subscription_ends_at_cancel_at_period_end
- Fix: implement handle_payment_failed() to set subscription_status=PAST_DUE
- Fix: harmonize .env.production Stripe variable names to match pricing_config.py
- Fix: add missing FRONTEND_URL and STRIPE_PUBLISHABLE_KEY to .env.production
- Add all Stripe Price IDs (test mode) to .env.production
- Wire credit purchase buttons to /api/v1/auth/create-credits-checkout
- Dashboard sync post-checkout was already implemented (no change needed)

Stripe test keys: configured in .env.production
Webhook: must be configured on server via stripe CLI or Stripe Dashboard
Webhook URL: https://wordly.art/api/v1/auth/webhook/stripe

.env.production

@@ -15,6 +15,7 @@ LOG_LEVEL=INFO
 # ---- Domaine ----
 DOMAIN=wordly.art
 NEXT_PUBLIC_API_URL=https://wordly.art
+FRONTEND_URL=https://wordly.art
 BACKEND_PORT=8000
 FRONTEND_PORT=3000
 
@@ -91,8 +92,15 @@ GRAFANA_USER=admin
 GRAFANA_PASSWORD=WordlyGrafana2026!
 
 # ---- Stripe ----
-STRIPE_SECRET_KEY=
+# Clés TEST — remplacer par sk_live_... / pk_live_... en production réelle
+STRIPE_PUBLISHABLE_KEY=pk_test_51SkSHkCKXUJE51jnCr2QV4vCE18GH1XF59eHrHOV46EORuZTPVXFXxrbcJamyoJLaUHMc2McCRVkU4b6VMAWVR2R00XRdwmXMx
+STRIPE_SECRET_KEY=sk_test_51SkSHkCKXUJE51jnbEtXZ0nKiTHTa8ohDwLH8fZiDVEx6Ze0g5dg4fGJJgX1VgNHvF93GE3HTramT3oQrCaqOxid00OXTcZlsW
 STRIPE_WEBHOOK_SECRET=
-STRIPE_STARTER_PRICE_ID=
-STRIPE_PRO_PRICE_ID=
-STRIPE_BUSINESS_PRICE_ID=
+
+# Price IDs créés dans Stripe Dashboard (wordly.art — test mode)
+STRIPE_PRICE_STARTER_MONTHLY=price_1TdF8FCKXUJE51jnNAeLqhF3
+STRIPE_PRICE_STARTER_YEARLY=price_1TdF8GCKXUJE51jnBhVVrjrh
+STRIPE_PRICE_PRO_MONTHLY=price_1TdF8GCKXUJE51jn9ChAAhKM
+STRIPE_PRICE_PRO_YEARLY=price_1TdF8HCKXUJE51jnpsvBivDe
+STRIPE_PRICE_BUSINESS_MONTHLY=price_1TdF8HCKXUJE51jn2K9EeBGJ
+STRIPE_PRICE_BUSINESS_YEARLY=price_1TdF8ICKXUJE51jnmgWZxW4U