Performance and response time monitoring

We need to know which requests take time, which DB queries are slow, what consumes memory, and how these change over time

## Links to dig

### Free OS hostable tools
- Prometheus / Grafana
- https://jolicode.com/blog/comment-monitorer-rapidement-du-code-php
- https://tsh.io/blog/grafana-and-prometheus-metrics/
- https://blog.zwindler.fr/2020/04/13/decouvrir-prometheus-et-grafana-par-lexemple/

### SaaS services

- https://docs.sentry.io/platforms/php/guides/symfony/performance/
- https://www.blackfire.io/
- https://www.atatus.com/for/symfony
- https://www.dynatrace.com/technologies/php-monitoring/symfony/

The issue names no files, tests, or entry points. Start by reviewing the linked Prometheus, Grafana, Sentry, Blackfire, Atatus, and Dynatrace resources and determining which approach fits this Symfony backend. Done should cover request timing, slow database queries, memory usage, and their changes over time, but the issue does not define an implementation scope.

Written by the indexing model from the issue text.