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

database/models.py

@@ -82,6 +82,8 @@ class User(Base):
 
     stripe_customer_id = Column(String(255), nullable=True, index=True)
     stripe_subscription_id = Column(String(255), nullable=True)
+    subscription_ends_at = Column(DateTime, nullable=True)
+    cancel_at_period_end = Column(Boolean, default=False, nullable=False)
 
     reset_token = Column(String(255), nullable=True)
     reset_token_expires = Column(DateTime, nullable=True)