Fix Docker signup, refresh docs, and improve blank invoice flow.

Parse DISABLE_SIGNUPS and related env booleans correctly for Compose string values, and derive auth trustedOrigins from BETTER_AUTH_URL. Rewrite README and architecture docs with the git.soconnor.dev remote and accurate deployment guidance. Allow zero-line-item draft invoices with validation when sending email.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -1,5 +1,6 @@
-# Copy this file to .env before running Docker Compose:
-# cp .env.example .env
+# beenvoice-web environment
+# Local dev:  cp .env.example .env.local
+# Docker:     cp .env.example .env
 
 # Runtime
 NODE_ENV=production
@@ -43,6 +44,9 @@ RESEND_DOMAIN=
 NEXT_PUBLIC_UMAMI_WEBSITE_ID=
 NEXT_PUBLIC_UMAMI_SCRIPT_URL=https://analytics.umami.is/script.js
 
+# Block new email/password registrations (optional)
+# DISABLE_SIGNUPS=true
+
 # SSO via Authentik OIDC (optional)
 NEXT_PUBLIC_AUTHENTIK_ENABLED=false
 AUTHENTIK_ISSUER=