news-digest: per-instance cron schedule via env

Hardcoded crontab → render at container start from
DIGEST_CRON_AM + DIGEST_CRON_PM. Defaults match the original
0800 / 2000 so existing deploys are no-ops.

scripts/add-digest-user.sh learns --am and --pm flags so each
teammate's stack can fire on their hours:

  scripts/add-digest-user.sh bob --am "0 6 * * *" --pm "0 17 * * *"
  scripts/add-digest-user.sh carol --pm "30 18 * * 1-5"   # weekdays only

Standard 5-field cron syntax; busybox crond honors the container's
\$TZ. Removed the now-unused stacks/news-digest/crontab file and
the matching COPY in the Dockerfile.

stacks/news-digest/entrypoint.sh

@@ -29,8 +29,21 @@ if [ ! -f /output/index.html ]; then
         echo "[entrypoint] first run failed; cron will retry on schedule"
 fi
 
-# Tee crontab into the place busybox expects (already done in image
-# via COPY) and run crond in foreground. -L /dev/stdout sends cron
-# stdout/stderr to docker logs.
-echo "[entrypoint] starting crond (fires per /etc/crontabs/root)"
+# Render the crontab from env so each per-user instance can fire on
+# its own schedule. Defaults match the original singleton (0800 / 2000
+# local). Two separate vars (AM / PM) instead of one combined string
+# so users can tweak one fire without re-deriving the other.
+CRON_AM="${DIGEST_CRON_AM:-0 8 * * *}"
+CRON_PM="${DIGEST_CRON_PM:-0 20 * * *}"
+mkdir -p /etc/crontabs
+{
+    echo "# news-digest cron — generated at container start from env."
+    echo "$CRON_AM /usr/local/bin/run-digest.sh"
+    echo "$CRON_PM /usr/local/bin/run-digest.sh"
+} > /etc/crontabs/root
+echo "[entrypoint] crontab:"
+sed 's/^/  /' /etc/crontabs/root
+
+# Foreground crond. -L /dev/stdout sends cron stdout/stderr to docker logs.
+echo "[entrypoint] starting crond"
 exec crond -f -L /dev/stdout -l 8