Create a Salsa CI stats dashboard

It would be nice if we had a dashboard that showed stats about Salsa CI usage, such as how many pipelines are running, how many packages have adopted it, and what are the most common failures. If the dashboard also had alerts, we could configure it to alarm if a large number of jobs start failing (suggesting a regression in the pipeline).

This would need an extensive proposal how stats are collected and stored. See some discussion mailing list threads https://alioth-lists.debian.net/pipermail/debian-salsa-ci/2024-September/000199.html and https://alioth-lists.debian.net/pipermail/debian-salsa-ci/2024-August/000167.html