Rewrite from Go + vanilla JS to Python (FastAPI) + React (TypeScript)

- 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

backend/requirements.txt

@@ -0,0 +1,11 @@
+fastapi==0.109.0
+uvicorn[standard]==0.27.0
+sqlalchemy==2.0.25
+psycopg2-binary==2.9.9
+alembic==1.13.1
+boto3==1.34.25
+python-multipart==0.0.6
+pydantic==2.5.3
+pydantic-settings==2.1.0
+python-jose[cryptography]==3.3.0
+passlib[bcrypt]==1.7.4