docs: surface DISABLE_REGISTRATION self-hosting flag (#11)

DISABLE_REGISTRATION already closes public self-service signup (first-user
setup on an empty DB still allowed) and the login page already hides its
"Create account" button when it's set - but the flag was easy to miss: it was
in the README env-var table yet absent from .env.example (the file
self-hosters actually copy) and from the README systemd unit example.

- .env.example: document DISABLE_REGISTRATION + DISABLE_HOMEPAGE under the
  Self-hosting section.
- README: add commented Environment= lines for both to the systemd example,
  noting the login UI hides the signup button to match.

Docs only - no code change. Backend gate (routes/auth.js canRegister +
/auth/config registration_enabled) and the login.js hiding already behave
correctly; verified registration_enabled flips to false under the flag.

Closes #11.

.env.example

@@ -11,6 +11,17 @@
 # instance never emits mail from a domain that isn't yours.
 SELF_HOSTED=true
 
+# Close public self-service registration — for instances where all accounts are
+# provisioned by your team (admin "Add user" / invites). When true, the public
+# signup route is blocked (OAuth auto-signup with it) AND the login page hides
+# its "Create account" button so the UI matches the backend. First-user setup on
+# an empty DB is still allowed so a fresh install can be initialized.
+# DISABLE_REGISTRATION=true
+
+# Redirect "/" to the app (/app) instead of serving the marketing landing page.
+# For internal-only deployments that don't want the public homepage shown.
+# DISABLE_HOMEPAGE=true
+
 # Where new-signup admin notifications are sent. Leave UNSET to disable admin
 # notifications entirely — the user's welcome email is unaffected. Self-hosters
 # who want to be notified of signups set this to their own address.

README.md

@@ -243,6 +243,13 @@ Restart=always
 Environment=PORT=3001
 Environment=NODE_ENV=production
 Environment=SELF_HOSTED=true
+# Lock down an internal / provisioned-only instance (all accounts created by your
+# team). DISABLE_REGISTRATION closes self-service signup — first-user setup on an
+# empty DB is still allowed, and the login page hides its "Create account" button
+# to match. DISABLE_HOMEPAGE sends `/` straight to the app instead of the
+# marketing landing page.
+# Environment=DISABLE_REGISTRATION=true
+# Environment=DISABLE_HOMEPAGE=true
 # Environment=APP_URL=https://signage.yourcompany.com
 # Environment=STRIPE_SECRET_KEY=sk_live_...
 # Environment=STRIPE_WEBHOOK_SECRET=whsec_...