Add cron container for daily backups and content export

- Add cron service to docker-compose with backup (3 AM) and export (4 AM) schedules
- Remove redundant content/articles/ and content/curriculum/ (now in Outline, exported to content/wiki/)
- Fix env var mismatch: support both OUTLINE_API_KEY and OUTLINE_API_TOKEN
- Drop updatedAt from export frontmatter to reduce noisy commits
- Add backups/ to gitignore

cron/Dockerfile

@@ -0,0 +1,19 @@
+FROM alpine:3.20
+
+RUN apk add --no-cache \
+    bash \
+    docker-cli \
+    git \
+    nodejs \
+    npm \
+    openssh-client \
+    gzip
+
+WORKDIR /app
+
+COPY crontab /etc/crontabs/root
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+ENTRYPOINT ["/entrypoint.sh"]
+CMD ["crond", "-f", "-l", "2"]