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/MonitorCacheKeys.php

@@ -67,6 +67,7 @@ final readonly class MonitorCacheKeys implements CacheItemPoolInterface
         $this->cache->save($changeList);
     }
 
+    /** @throws InvalidArgumentException */
     public function getItem(string $key): CacheItemInterface
     {
         return $this->cache->getItem($key);
@@ -79,6 +80,7 @@ final readonly class MonitorCacheKeys implements CacheItemPoolInterface
         return $this->cache->getItems($keys);
     }
 
+    /** @throws InvalidArgumentException */
     public function hasItem(string $key): bool
     {
         return $this->cache->hasItem($key);
@@ -97,6 +99,7 @@ final readonly class MonitorCacheKeys implements CacheItemPoolInterface
         return true;
     }
 
+    /** @throws InvalidArgumentException */
     public function deleteItem(string $key): bool
     {
         $this->isValid($key);
@@ -113,6 +116,7 @@ final readonly class MonitorCacheKeys implements CacheItemPoolInterface
         return $this->cache->deleteItem($key);
     }
 
+    /** @throws InvalidArgumentException */
     public function deleteItems(array $keys): bool
     {
         $this->allValid($keys);
@@ -145,6 +149,7 @@ final readonly class MonitorCacheKeys implements CacheItemPoolInterface
         return $this->cache->saveDeferred($item);
     }
 
+    /** @throws InvalidArgumentException */
     public function commit(): bool
     {
         return $this->cache->commit();