Deploy on DigitalOcean App Platform (GitHub-source build) + consolidate audit-fixes

Deploy config:
- .do/app.yaml: build the Dockerfile directly from GitHub (deploy_on_push) instead
  of a pre-built DOCR image; NEXT_PUBLIC_* set RUN_AND_BUILD_TIME with the
  propertymanagement.network domain so they bake into the client bundle; add
  custom domains block (apex + www); wire Sentry DSN (server + browser).

Included pending work from the audit-fixes branch:
- AI provider abstraction (OpenAI/Anthropic, admin-selectable; Anthropic default)
- Per-landlord e-signature (DocuSign OAuth + Dropbox Sign) + migration 0010
- Outbound webhooks / Zapier integration
- PayPal removal (Stripe-only billing)
- Storage hardening (fail-loud when Spaces unconfigured), security fixes

Verified: full production Docker build (same build-args as DO) passes clean.

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

app/actions/auth.ts

@@ -36,7 +36,12 @@ export async function signUp(formData: FormData) {
       headers: h,
     })
   } catch (e) {
-    const msg = e instanceof APIError ? e.message : "Sign up failed"
+    const raw = e instanceof APIError ? e.message : "Sign up failed"
+    // Don't reveal that an email is already registered (user enumeration) — the
+    // "already exists" path must not be distinguishable from other failures.
+    const msg = /exist|registered|already|taken/i.test(raw)
+      ? "We couldn't complete your sign-up. Please try a different email or sign in."
+      : raw
     redirect(`/signup?error=${encodeURIComponent(msg)}`)
   }