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>

src/bootstrap.php

@@ -56,7 +56,7 @@ $mailer = $config->mail->transport === 'log'
     : new PhpMailerMailer($config->mail);
 $verifier = new EmailVerifier($verificationTokens, $users, $mailer, $config->appUrl);
 
-$authController = new AuthController($users, $session, $verifier);
+$authController = new AuthController($users, $session, $verifier, $config->allowRegistration);
 $emailController = new EmailVerificationController($users, $verificationTokens, $verifier, $session);
 $projectController = new ProjectController($projects, $cardStatuses);
 $cardController = new CardController($projects, $cards, $cardStatuses);