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/Support/Config.php

@@ -15,6 +15,8 @@ final class Config
         public readonly string $jwtSecret,
         public readonly int $jwtTtl,
         public readonly bool $displayErrors,
+        /** When false, POST /auth/magic-link only signs existing users in -- it never creates a new account. */
+        public readonly bool $allowRegistration,
         /** Base URL of the frontend, used to build magic links. */
         public readonly string $appUrl,
         public readonly MailConfig $mail,
@@ -40,6 +42,7 @@ final class Config
         $jwtSecret = self::env('JWT_SECRET') ?? self::resolveSecret($storagePath . '/secret.key');
         $jwtTtl = (int) (self::env('JWT_TTL') ?? '86400');
         $displayErrors = filter_var(self::env('APP_DEBUG', 'false'), FILTER_VALIDATE_BOOL);
+        $allowRegistration = filter_var(self::env('APP_ALLOW_REGISTRATION', 'true'), FILTER_VALIDATE_BOOL);
 
         $appUrl = rtrim(self::env('APP_URL', 'http://localhost:5173'), '/');
 
@@ -60,7 +63,7 @@ final class Config
             smtpEncryption: strtolower(self::env('MAIL_SMTP_ENCRYPTION', 'tls')),
         );
 
-        return new self($databasePath, $jwtSecret, $jwtTtl, $displayErrors, $appUrl, $mail);
+        return new self($databasePath, $jwtSecret, $jwtTtl, $displayErrors, $allowRegistration, $appUrl, $mail);
     }
 
     private static function env(string $key, ?string $default = null): ?string