feat(billing): add subscription and Stripe billing router

- Add stripeCustomerId column to users table
- Create Stripe client initialization (web/src/server/stripe.ts)
- Add billing service with getOrCreateCustomer, checkout/portal sessions,
  subscription management, invoice listing, and webhook event handling
- Create billing tRPC router with getSubscription, createCheckoutSession,
  createPortalSession, cancelSubscription, reactivateSubscription, listInvoices
- Add raw webhook endpoint at /api/stripe/webhook with signature verification
- Define Valibot schemas for all billing procedure inputs
- Wire billing router into root tRPC router
- Update schema tests for new column/index counts
- Write unit tests for billing service and router

web/.env.example

@@ -1 +1,8 @@
 DATABASE_URL="postgresql://postgres:postgres@localhost:5432/shieldai"
+
+# Stripe (get test keys from https://dashboard.stripe.com/test/apikeys)
+STRIPE_SECRET_KEY="sk_test_..."
+STRIPE_WEBHOOK_SECRET="whsec_..."
+STRIPE_PRICE_BASIC="price_basic"
+STRIPE_PRICE_PLUS="price_plus"
+STRIPE_PRICE_PREMIUM="price_premium"