feat(deploy): AWS SAM template + Makefile + GitHub Actions

- AWS SAM CloudFormation template for Lambda + DynamoDB + EventBridge
- SAM config for us-east-1 deployment with guided parameters
- Unified Makefile: build-lambda, dynamodb-local, sam-* targets
- GitHub Actions: OIDC trust + SAM deploy on push to main
- CI job: add iac stage (sam validate)
- .gitignore: build/, bin/, .aws-sam/, samconfig.local.toml

.github/workflows/ci.yml

@@ -64,3 +64,14 @@ jobs:
 
       - name: docker build
         run: docker build -t miti99bot-go .
+
+  iac:
+    name: SAM template validate
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: aws-actions/setup-sam@v2
+        with:
+          use-installer: true
+      - name: sam validate (offline)
+        run: sam validate --lint --region ap-southeast-1