Restructure project layout and add ECR repository CloudFormation template

- Move application entrypoint to cmd/lambda/
- Move Dockerfile to docker/ for clearer build context separation
- Promote go.mod/go.sum to project root
- Move CloudFormation templates under infra/cfn/ for consistent infra layout
- Add new template-container-repository.yaml defining ECR repository (blog-deployment)
- Move Lambda test files to test/ directory

infra/cfn/template-container-repository.yaml

@@ -0,0 +1,21 @@
+AWSTemplateFormatVersion: "2010-09-09"
+
+Parameters:
+  RepositoryName:
+    Type: String
+    Default: blog-deployment
+
+Resources:
+  # ECR Repository
+  Repository:
+    Type: AWS::ECR::Repository
+    DeletionPolicy: Retain
+    Properties:
+      RepositoryName: !Ref RepositoryName
+      ImageScanningConfiguration:
+        ScanOnPush: true
+
+Outputs:
+  RepositoryUri:
+    Description: URI of the ECR repository
+    Value: !GetAtt Repository.RepositoryUri