chore: PHP 8.1+ 요구사항 및 의존성 잠금

- composer.json: PHP >= 8.1 요구
- composer.json: platform php 8.1.0 설정 추가
- composer.lock: 의존성 버전 잠금
- .gitignore: vendor 디렉토리 제외

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

extend/r2-storage/.gitignore

@@ -0,0 +1 @@
+/vendor/

extend/r2-storage/composer.json

@@ -4,8 +4,13 @@
     "type": "library",
     "license": "MIT",
     "require": {
-        "php": ">=7.4",
-        "aws/aws-sdk-php": "^3.0"
+        "php": ">=8.1",
+        "aws/aws-sdk-php": "^3.300"
+    },
+    "config": {
+        "platform": {
+            "php": "8.1.0"
+        }
     },
     "autoload": {
         "psr-4": {