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/compose.yaml

@@ -48,6 +48,10 @@ services:
       - DIGEST_MINIFLUX_LOCAL_CATEGORY=${DIGEST_MINIFLUX_LOCAL_CATEGORY:-Local}
       - DIGEST_MINIFLUX_HEADLINES_HOURS=${DIGEST_MINIFLUX_HEADLINES_HOURS:-8}
       - DIGEST_MINIFLUX_HEADLINES_MAX=${DIGEST_MINIFLUX_HEADLINES_MAX:-15}
+      # Per-instance cron schedule. Standard 5-field syntax. busybox
+      # crond honors $TZ. Defaults preserve the original 0800 / 2000.
+      - DIGEST_CRON_AM=${DIGEST_CRON_AM:-0 8 * * *}
+      - DIGEST_CRON_PM=${DIGEST_CRON_PM:-0 20 * * *}
     volumes:
       - ${NEWS_DIGEST_OUTPUT_DIR}:/output
     networks: