PATCH: Send Rubika change alerts after each category

Changed file:
- cron_sync_iranskmei.php

Behavior:
- Alerts are now buffered only for the current category.
- At CAT_END, before the cron moves to the next category, the bot sends that category's alerts to all Rubika subscribers.
- After sending, the category alert buffer is cleared.
- Final OK/cron log still reports the total alert count across all categories.

Trace log markers:
- CAT_END slug=<slug> products=<count> alerts=<category_alert_count>
- CAT_ALERTS_SENT slug=<slug> alerts=<category_alert_count>