Add Docker support

- Dockerfile: multi-stage Python 3.12 slim build
- docker-compose.yml: app + PostgreSQL 16 with healthcheck
- requirements.txt: fastapi, uvicorn, psycopg, passlib, jose
- .dockerignore: exclude venv, pycache, secrets
- app.py: env-based DB config (DB_HOST/PORT/NAME/USER/PASSWORD)
- README: Docker deploy instructions

requirements.txt

@@ -0,0 +1,6 @@
+fastapi>=0.100.0
+uvicorn[standard]>=0.23.0
+psycopg[binary]>=3.1.0
+passlib[bcrypt]>=1.7.4
+python-jose[cryptography]>=3.3.0
+python-multipart>=0.0.6