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/(admin)/admin/system/page.tsx

@@ -1,4 +1,6 @@
 import { getSystemCounts, getEnvHealth } from "@/lib/db/admin-queries"
+import { getMaintenanceMode } from "@/lib/settings"
+import { MaintenanceToggle } from "@/components/admin/maintenance-toggle"
 import { formatDate } from "@/lib/utils"
 import { Settings, Database, Table2 } from "lucide-react"
 
@@ -11,9 +13,10 @@ function humanize(name: string): string {
 }
 
 export default async function AdminSystemPage() {
-  const [{ counts, cronLastRun }, env] = await Promise.all([
+  const [{ counts, cronLastRun }, env, maintenance] = await Promise.all([
     getSystemCounts(),
     Promise.resolve(getEnvHealth()),
+    getMaintenanceMode(),
   ])
 
   return (
@@ -26,6 +29,14 @@ export default async function AdminSystemPage() {
         </p>
       </div>
 
+      {/* Maintenance mode control */}
+      <div className="mb-5">
+        <MaintenanceToggle
+          initialEnabled={maintenance.enabled}
+          initialMessage={maintenance.message ?? ""}
+        />
+      </div>
+
       <div className="grid grid-cols-1 lg:grid-cols-2 gap-4 sm:gap-5 mb-5">
         {/* ── Environment configuration ───────────────────────────────── */}
         <div className="rounded-2xl border border-white/[0.06] bg-[#16161f] overflow-hidden">