Enable password self-registration and keep SSO off (#81)

## Summary
- Closes #80.
- Password self-registration is on by default (`ENABLE_ACCOUNT_REGISTRATION`). `/user/create/` issues tokens and `needs_checkout`, and the web sign-up page already redirects into Stripe Checkout.
- Browser SSO stays off (`ENABLE_SSO`, default false). Live Google and Microsoft client ids stay available for Drive linking, but public settings no longer advertise them, and login/signup OAuth start and callback are rejected.

Stripe path checked before this change: Checkout builds a subscription session (Founders at $10/month, or `STRIPE_PRICE_ID`), stores a draft invoice, and signed `checkout.session.completed` / `invoice.paid` webhooks grant the plan. Checkout, webhook, portal, registration, and OAuth tests passed.

## Deploy note
If `chat_backend_prod.env` or `chat_backend_beta.env` on the control node still sets `ENABLE_ACCOUNT_REGISTRATION=false`, that line overrides this default. Set it to `true` or remove it, and leave `ENABLE_SSO` unset or `false`.

## Test plan
- [ ] `GET /api/public/settings/` returns `enable_account_registration: true` and `oauth.google` / `oauth.microsoft` false.
- [ ] Sign-up form creates an account and redirects to Stripe Checkout. No Google or Microsoft button.
- [ ] Checkout success grants the Founders plan after the webhook.
- [ ] `/api/auth/oauth/google/start/?intent=signup` returns 403. Drive connect still starts.
- [ ] Setting `ENABLE_ACCOUNT_REGISTRATION=false` closes `/user/create/` again.

Reviewed-on: #81

.env.prod.example

@@ -59,9 +59,13 @@ EMAIL_USE_TLS=true
 # Captcha
 CAPTCHA_SECRET_KEY=replace-with-captcha-secret
 
-# Self-serve account registration (sign-up). Keep false until ready to open
-# public registration; set true in chat_backend_prod.env / chat_backend_beta.env.
-ENABLE_ACCOUNT_REGISTRATION=false
+# Password self-serve sign-up. Default true. Set false in
+# chat_backend_prod.env / chat_backend_beta.env to close public registration.
+# An explicit false in that secret overrides the code default.
+ENABLE_ACCOUNT_REGISTRATION=true
+# Browser SSO stays off. OAuth client ids below are for Drive linking until
+# this is set true.
+ENABLE_SSO=false
 
 # OAuth SSO — Google / Microsoft (#24). Never commit real secrets.
 # Register redirect URIs: