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>

db/migrations/017-billing-enhancements.sql

@@ -0,0 +1,27 @@
+-- Migration 017: Billing Enhancements
+-- Adds support for annual billing, free trials, ACH/invoice billing,
+-- and past_due grace period status.
+
+-- ============================================================================
+-- 1. Add billing_interval column (month or year)
+-- ============================================================================
+ALTER TABLE shared.organizations ADD COLUMN IF NOT EXISTS billing_interval VARCHAR(20) DEFAULT 'month';
+
+-- ============================================================================
+-- 2. Add collection_method column (charge_automatically or send_invoice)
+-- ============================================================================
+ALTER TABLE shared.organizations ADD COLUMN IF NOT EXISTS collection_method VARCHAR(20) DEFAULT 'charge_automatically';
+
+-- ============================================================================
+-- 3. Update status CHECK to include 'past_due'
+-- ============================================================================
+ALTER TABLE shared.organizations DROP CONSTRAINT IF EXISTS organizations_status_check;
+ALTER TABLE shared.organizations ADD CONSTRAINT organizations_status_check
+  CHECK (status IN ('active', 'suspended', 'trial', 'archived', 'past_due'));
+
+-- ============================================================================
+-- 4. Ensure plan_level CHECK includes SaaS tiers (idempotent with 015)
+-- ============================================================================
+ALTER TABLE shared.organizations DROP CONSTRAINT IF EXISTS organizations_plan_level_check;
+ALTER TABLE shared.organizations ADD CONSTRAINT organizations_plan_level_check
+  CHECK (plan_level IN ('standard', 'premium', 'enterprise', 'starter', 'professional'));