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

.gitignore

@@ -5,4 +5,5 @@ __pycache__/
 .env
 *.log
 .DS_Store
+.dbpass
 migrations/_migrations*