- Add checksum.py module with SHA256 utilities and HashingStreamWrapper
- Add ChecksumMismatchError and VerifyingStreamWrapper for verification
- Add get_verified() and get_stream_verified() to storage layer
- Add verify query param and verify_mode (pre/stream) to download API
- Add checksum headers: X-Checksum-SHA256, Digest, ETag, X-Verified
- Add comprehensive unit tests for checksum utilities
- Add integration tests for download verification API
- 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