Add Listener for missing db indices

[Grotax]

• Resolves: 25.0.0-alpha3: missing index on feeds(deleted_at) #2526

Summary

This should in theory allow admins to get the notification of missing indices and the option to add them via
occ db:add-missing-indices

tested:

• changed migration creates the new index on first install
  

• occ db:add-missing-indices is offered in admin overview after upgrade
  

root@5a630161f41e:/var/www/html# sudo -u www-data php ./occ db:add-missing-indices          
Adding additional news_feeds_deleted_at_index index to the oc_news_feeds table, this can take some time...
oc_news_feeds table updated successfully.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Changelog entry added for all important changes.

[Grotax]

got it working :) also fixed some other small things that I stumbled on during testing