Add a config toggle to turn off new user registration

APP_ALLOW_REGISTRATION (default true) gates the only "sign up" this app has --
the account-creation side effect of POST /api/auth/magic-link. When false, an
unknown address is silently ignored (find-only, no findOrCreateByEmail) while
an existing address still gets its sign-in link as normal; the response is
identical either way (202, same message), so there's still no enumeration
signal.

- Config::allowRegistration, read from APP_ALLOW_REGISTRATION.
- AuthController::requestLoginLink takes the flag; only looks up (doesn't
  create) when it's off.
- docker-compose.yml / .env.example / README document the new var.
- ApiTestCase::reconfigure() rebuilds the app against changed env (same
  database) for tests that need a non-default Config; two new AuthTest
  cases cover both halves (blocks a new address, doesn't block an existing
  one). 59 tests pass.

Verified against the rebuilt container: with the flag on (default), a new
address gets a link and an account; switched off via the same env var, a
brand-new address gets the same 202 but no email and no user row, while an
address that already had an account still receives its link.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.yml

@@ -14,6 +14,9 @@ services:
       # Leave blank to auto-generate a secret into the storage volume on first run.
       JWT_SECRET: "${JWT_SECRET:-}"
       JWT_TTL: "${JWT_TTL:-86400}"
+      # Set to false to stop new accounts being created (existing users can
+      # still sign in).
+      APP_ALLOW_REGISTRATION: "${APP_ALLOW_REGISTRATION:-true}"
       # The SPA and the API are both served from this container.
       APP_URL: "${APP_URL:-http://localhost:8080}"
       # Deliver to the Mailpit catcher below; read mail at http://localhost:8025.