Add daily stats collection cron job at 05:00 UTC

The gniza stats command is now automatically scheduled when
install_schedules() runs, tagged as # gniza:_stats so it is
managed alongside backup schedules (installed/removed together).

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

lib/schedule.sh

@@ -206,6 +206,10 @@ install_schedules() {
         filtered+="$line"$'\n'
     done <<< "$current_crontab"
 
+    # Append daily stats collection (runs at 05:00 UTC)
+    new_lines+="${GNIZA_CRON_TAG}_stats"$'\n'
+    new_lines+="0 5 * * * /usr/local/bin/gniza stats >> /var/log/gniza/cron-stats.log 2>&1"$'\n'
+
     # Append new lines
     local final="${filtered}${new_lines}"