feat(ci): auto-run alembic migrations and seed on container start

- Add entrypoint.sh (migrate → seed → uvicorn)
- Add python-dotenv to requirements.txt
- Backend container uses entrypoint instead of direct uvicorn

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

Containerfile.backend

@@ -20,4 +20,4 @@ ENV PYTHONUNBUFFERED=1
 
 EXPOSE 8000
 
-CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]
+CMD ["sh", "scripts/entrypoint.sh"]