feat: refactor to SI Quiz with Space Invaders arcade interface

- Updated Dockerfile to Python 3.11, selective COPY for cleaner image
- Expanded .gitignore with Python/OS patterns
- Added config.json, questions.json, env_example, and static assets
- Updated templates (host, player, scores) with Space Invaders UI
- Rewrote README for SI Quiz branding and setup docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

env_example

@@ -0,0 +1,10 @@
+# SI Quiz — Environment Variables
+# Copy this file to .env and adjust as needed.
+# docker-compose.yml reads these automatically.
+
+# Path to the SQLite database (inside the container)
+DB_PATH=/data/si-quiz.db
+
+# Path to config and question files (relative to /app inside container)
+CONFIG_PATH=config.json
+QUESTIONS_PATH=questions.json