feat: add annual billing, free trial, upgrade/downgrade, and ACH invoice support

- Add monthly/annual billing toggle with 25% annual discount on pricing page
- Implement 14-day no-card free trial (server-side Stripe subscription creation)
- Enable upgrade/downgrade via Stripe Customer Portal
- Add admin-initiated ACH/invoice billing for enterprise customers
- Add billing card to Settings page with plan info and Manage Billing button
- Handle past_due status with read-only grace period access
- Add trial ending and trial expired email templates
- Add DB migration for billing_interval and collection_method columns
- Update ONBOARDING-AND-AUTH.md documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -13,6 +13,30 @@ AI_MODEL=qwen/qwen3.5-397b-a17b
 # Set to 'true' to enable detailed AI prompt/response logging
 AI_DEBUG=false
 
+# Stripe Billing
+STRIPE_SECRET_KEY=sk_test_your_stripe_secret_key
+STRIPE_WEBHOOK_SECRET=whsec_your_webhook_secret
+
+# Stripe Price IDs (Monthly)
+STRIPE_STARTER_MONTHLY_PRICE_ID=price_starter_monthly
+STRIPE_PROFESSIONAL_MONTHLY_PRICE_ID=price_professional_monthly
+STRIPE_ENTERPRISE_MONTHLY_PRICE_ID=price_enterprise_monthly
+
+# Stripe Price IDs (Annual — 25% discount)
+STRIPE_STARTER_ANNUAL_PRICE_ID=price_starter_annual
+STRIPE_PROFESSIONAL_ANNUAL_PRICE_ID=price_professional_annual
+STRIPE_ENTERPRISE_ANNUAL_PRICE_ID=price_enterprise_annual
+
+# Trial configuration
+REQUIRE_PAYMENT_METHOD_FOR_TRIAL=false
+
+# Email (Resend)
+RESEND_API_KEY=re_your_resend_api_key
+
+# Application
+APP_URL=http://localhost
+INVITE_TOKEN_SECRET=dev-invite-secret
+
 # New Relic APM — set ENABLED=true and provide your license key to activate
 NEW_RELIC_ENABLED=false
 NEW_RELIC_LICENSE_KEY=your_new_relic_license_key_here