Only pass explicit credentials to boto3 if they're actually set.
This allows the default credential chain (including IRSA web identity
tokens) to be used when no access key is configured.
Also adds CHANGELOG entries for AWS services configuration.
- Backend: Python 3.12 with FastAPI, SQLAlchemy, boto3
- Frontend: React 18 with TypeScript, Vite build tooling
- Updated Dockerfile for multi-stage Node + Python build
- Updated CI pipeline for Python backend
- Removed old Go code (cmd/, internal/, go.mod, go.sum)
- Updated README with new tech stack documentation
