Consolidate audit-fixes branch: webhooks, integrations, and deploy hardening

Batch commit of the pending working tree on security/audit-fixes-2026-07.
Major areas:
- Outbound webhooks / Zapier: schema + signed delivery with retries, public
  v1 API (REST-hook subscribe/unsubscribe), settings UI, cron drain.
- Deploy hardening: email via SMTP2GO (Resend fully removed), verified DB TLS
  (DATABASE_SSL=require + DATABASE_CA), storage fails loud in production when
  Spaces is unconfigured instead of silently using ephemeral disk.
- Integrations & features (concurrent work): accounting (QuickBooks/Xero),
  e-signature (DocuSign/Dropbox Sign), PayPal, geocoding/maps, onboarding,
  expanded legal pages.
- DB migrations 0006–0009.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/cron/daily/route.ts

@@ -7,7 +7,7 @@ import { formatCurrency, formatDate, daysUntil } from "@/lib/utils"
 import { isAuthorizedCron } from "@/lib/cron-auth"
 
 // Combined daily cron: rent reminders + overdue marking + lease expiry emails
-// Runs daily at 9am (see vercel.json)
+// Runs daily at 9am UTC (scheduled via DigitalOcean Functions — see DIGITALOCEAN.md)
 export async function GET(request: Request) {
   if (!isAuthorizedCron(request)) {
     return NextResponse.json({ error: "Unauthorized" }, { status: 401 })