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/Service/BackupCodeManager.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Service;
 
+use App\AppConstants;
 use App\MonitorCacheKeys;
 use App\Trait\HasLoggerTrait;
 use App\Trait\StringTrait;
@@ -84,7 +85,7 @@ final readonly class BackupCodeManager implements BackupCodeInterface
              * we want this to keep forever, so a few hundred years should do it */
             $backupItem->expiresAt(DateTimeImmutable::createFromFormat(
                 'Y-m-d',
-                '2999-12-31'
+                AppConstants::FAR_FUTURE_DATE
             ));
             $this->sessionCache->save($backupItem);
 
@@ -104,7 +105,7 @@ final readonly class BackupCodeManager implements BackupCodeInterface
              * we want this to keep forever, so a few hundred years should do it */
             $backupItem->expiresAt(DateTimeImmutable::createFromFormat(
                 'Y-m-d',
-                '2999-12-31'
+                AppConstants::FAR_FUTURE_DATE
             ));
             $this->sessionCache->saveDeferred($backupItem);
         }