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>

tests/ApiTestCase.php

@@ -70,6 +70,18 @@ abstract class ApiTestCase extends TestCase
         }
     }
 
+    /**
+     * Change config env vars and rebuild the app against them (same database).
+     * For settings Config::load() only reads once at boot, e.g. APP_ALLOW_REGISTRATION.
+     *
+     * @param array<string, string> $vars
+     */
+    protected function reconfigure(array $vars): void
+    {
+        $this->setEnv($vars);
+        $this->app = require dirname(__DIR__) . '/src/bootstrap.php';
+    }
+
     /** A connection to the test database, for seeding rows directly. */
     protected function db(): PDO
     {