Add Dockerfile and .dockerignore for backend container image

[echska]

Motivation

• Provide a reproducible container image for the backend service and reduce image size by excluding unnecessary files from the build context.

Description

• Add a .dockerignore file that excludes VCS, Python caches, frontend node_modules and build output, and other unnecessary files from the Docker build context.
• Add a Dockerfile based on python:3.12-slim that sets PYTHONDONTWRITEBYTECODE and PYTHONUNBUFFERED, installs dependencies from parental-control-system/backend/requirements.txt, copies the backend source, creates a non-root appuser, exposes port 5000, and starts the app with gunicorn using CMD ["sh", "-c", "gunicorn --bind 0.0.0.0:${PORT:-5000} main:app --workers 2 --threads 4 --timeout 60"].

Testing

• No automated tests were run for this change.

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Codex Task