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>

backend/src/modules/email/email.service.ts

@@ -96,6 +96,42 @@ export class EmailService {
     await this.send(email, subject, html, 'invite_member', { orgName, inviteUrl });
   }
 
+  async sendTrialEndingEmail(email: string, businessName: string, daysRemaining: number, settingsUrl: string): Promise<void> {
+    const subject = `Your free trial ends in ${daysRemaining} days — ${businessName}`;
+    const html = this.buildTemplate({
+      preheader: `Your HOA LedgerIQ trial for ${businessName} is ending soon.`,
+      heading: `Your Trial Ends in ${daysRemaining} Days`,
+      body: `
+        <p>Your free trial for <strong>${this.esc(businessName)}</strong> on HOA LedgerIQ ends in <strong>${daysRemaining} days</strong>.</p>
+        <p>To continue using all features without interruption, add a payment method before your trial expires.</p>
+        <p>If you don't add a payment method, your account will become read-only and you won't be able to make changes to your data.</p>
+      `,
+      ctaText: 'Add Payment Method',
+      ctaUrl: settingsUrl,
+      footer: 'If you have any questions about plans or pricing, just reply to this email.',
+    });
+
+    await this.send(email, subject, html, 'trial_ending', { businessName, daysRemaining, settingsUrl });
+  }
+
+  async sendTrialExpiredEmail(email: string, businessName: string): Promise<void> {
+    const appUrl = this.configService.get<string>('APP_URL') || 'https://app.hoaledgeriq.com';
+    const subject = `Your free trial has ended — ${businessName}`;
+    const html = this.buildTemplate({
+      preheader: `Your HOA LedgerIQ trial for ${businessName} has ended.`,
+      heading: 'Your Trial Has Ended',
+      body: `
+        <p>The free trial for <strong>${this.esc(businessName)}</strong> on HOA LedgerIQ has ended.</p>
+        <p>Your data is safe and your account is preserved. Subscribe to a plan to regain full access to your HOA financial management tools.</p>
+      `,
+      ctaText: 'Choose a Plan',
+      ctaUrl: `${appUrl}/pricing`,
+      footer: 'Your data will be preserved. You can reactivate your account at any time by subscribing to a plan.',
+    });
+
+    await this.send(email, subject, html, 'trial_expired', { businessName });
+  }
+
   async sendPasswordResetEmail(email: string, resetUrl: string): Promise<void> {
     const subject = 'Reset your HOA LedgerIQ password';
     const html = this.buildTemplate({