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/integrations/[provider]/callback/route.ts

@@ -0,0 +1,37 @@
+import { NextResponse } from "next/server"
+import { getProvider, saveConnection, type Provider } from "@/lib/accounting"
+import { verifyState } from "@/lib/accounting/state"
+
+// OAuth callback — exchanges the code for tokens and stores the connection.
+export async function GET(request: Request, { params }: { params: Promise<{ provider: string }> }) {
+  const { provider: pid } = await params
+  const prov = getProvider(pid)
+  const url = new URL(request.url)
+  const settings = new URL("/settings/integrations", request.url)
+
+  const code = url.searchParams.get("code")
+  const state = url.searchParams.get("state")
+  const realmId = url.searchParams.get("realmId") // QuickBooks includes this
+  const oauthError = url.searchParams.get("error")
+
+  if (oauthError || !prov) {
+    settings.searchParams.set("error", "connect_failed")
+    return NextResponse.redirect(settings)
+  }
+
+  const st = state ? verifyState(state) : null
+  if (!code || !st || st.provider !== pid) {
+    settings.searchParams.set("error", "invalid_state")
+    return NextResponse.redirect(settings)
+  }
+
+  try {
+    const tokens = await prov.exchangeCode(code, realmId)
+    if (!tokens.realmId) throw new Error("No organisation returned from provider")
+    await saveConnection(st.ownerId, pid as Provider, tokens)
+    settings.searchParams.set("connected", pid)
+  } catch {
+    settings.searchParams.set("error", "connect_failed")
+  }
+  return NextResponse.redirect(settings)
+}