Fix cron backups failing due to minimal PATH environment

Cron runs with a very limited PATH that may not include sshpass, rsync,
etc. Add explicit PATH to generated cron lines and log output to
cron.log instead of /dev/null for debugging.

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

lib/schedule.sh

@@ -165,7 +165,7 @@ build_cron_line() {
         extra_flags+=" --target=$SCHEDULE_TARGETS"
     fi
 
-    echo "$cron_expr $bin_path backup${extra_flags} >/dev/null 2>&1"
+    echo "$cron_expr PATH=\"/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin\" $bin_path backup${extra_flags} >>\"${LOG_DIR}/cron.log\" 2>&1"
 }
 
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