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/stripe/webhook/route.ts

@@ -19,11 +19,13 @@ export async function POST(request: Request) {
   }
 
   // Webhooks are not user-scoped: they identify the target row by the id /
-  // customer id stored in Stripe metadata. There is no RLS to bypass anymore.
+  // customer id stored in Stripe metadata. `user_id` is the current key;
+  // `supabase_user_id` is read as a fallback so subscriptions/checkouts created
+  // before the rename keep resolving. (Both hold the same app user id.)
   switch (event.type) {
     case "checkout.session.completed": {
       const session = event.data.object as Stripe.Checkout.Session
-      const userId = session.metadata?.supabase_user_id
+      const userId = session.metadata?.user_id ?? session.metadata?.supabase_user_id
       const plan = session.metadata?.plan
 
       if (!userId || !plan) break
@@ -41,7 +43,7 @@ export async function POST(request: Request) {
     case "customer.subscription.created":
     case "customer.subscription.updated": {
       const subscription = event.data.object as Stripe.Subscription
-      const userId = subscription.metadata?.supabase_user_id
+      const userId = subscription.metadata?.user_id ?? subscription.metadata?.supabase_user_id
       const plan = subscription.metadata?.plan
 
       if (!userId) break
@@ -62,7 +64,7 @@ export async function POST(request: Request) {
 
     case "customer.subscription.deleted": {
       const subscription = event.data.object as Stripe.Subscription
-      const userId = subscription.metadata?.supabase_user_id
+      const userId = subscription.metadata?.user_id ?? subscription.metadata?.supabase_user_id
 
       if (!userId) break