Revert to simple environment variable approach

- Remove backend pipeline deployment that was causing IAM issues
- Remove complex secret configuration from backend.ts
- Use simple environment variable approach from Amplify Console

🤖 Generated with [Claude Code](https://claude.ai/code)

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

amplify.yml

@@ -1,10 +1,4 @@
 version: 1
-backend:
-  phases:
-    build:
-      commands:
-        - npm ci --cache .npm
-        - npx ampx pipeline-deploy --branch $AWS_BRANCH --app-id $AWS_APP_ID
 frontend:
   phases:
     preBuild: