chore: nice-to-have improvements for v1.0

Code quality:
- Create AppConstants class with shared constants:
  - FAR_FUTURE_DATE (replaces duplicated '2999-12-31' strings)
  - MAX_INPUT_LENGTH (replaces duplicated 128 in Payload and StringTrait)
- Extract duplicated 'hi $id' response body into StringTrait::authSuccessResponse()
  method, used by AcceptListener, AllowListener, and LoginManager
- Add missing @throws InvalidArgumentException annotations to
  MonitorCacheKeys (getItem, hasItem, deleteItem, deleteItems, commit)

Configuration:
- Add proper env var type casting in services.yaml:
  - COOKIE_TTL → env(int:)
  - SUBDOMAIN_REDIRECT → env(bool:)
  - IP_TTL → env(int:)
  - TEAPOT → env(bool:)

Documentation:
- Create CONTRIBUTING.md with development setup, code style,
  testing guidelines, and PR process

src/Data/Payload.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Data;
 
+use App\AppConstants;
 use App\Enum\Scope;
 use Symfony\Component\HttpFoundation\InputBag;
 
@@ -59,11 +60,11 @@ final class Payload
 
         /* all input is limited */
         $payload = new Payload();
-        $payload->id    = mb_substr(trim($data->id), 0, 128);
-        $payload->nonce = mb_substr(trim($data->nonce), 0, 128);
+        $payload->id    = mb_substr(trim($data->id), 0, AppConstants::MAX_INPUT_LENGTH);
+        $payload->nonce = mb_substr(trim($data->nonce), 0, AppConstants::MAX_INPUT_LENGTH);
         $payload->json  = ($data->json ?? true);
         $payload->scope = Scope::tryFrom($data->scope ?? '') ?? Scope::Cookie;
-        $payload->token = mb_substr(trim($data->token), 0, 128);
+        $payload->token = mb_substr(trim($data->token), 0, AppConstants::MAX_INPUT_LENGTH);
 
         return Payload::constrict($payload);
     }