Possibility to delete events older than x via the API

**Describe what you are trying to accomplish and why in non technical terms**
I would like to delete all events older than X days or hours.

**Describe the solution you'd like**
I would find it very handy to be able to delete all events older than X days or hours via the API (or MQTT or in the web interface). I usually take a quick look at what has happened in the last few days and then the clips could actually be deleted.

**Describe alternatives you've considered**
I could change the retention period and restart Frigate. Then change back and restart again.

Start by tracing the existing event-deletion behavior through the API, MQTT integration, and web interface; the issue does not name specific files or tests. Confirm how retention-based deletion currently works, then define and verify deletion of events older than a caller-supplied number of days or hours without changing the retention setting.

Written by the indexing model from the issue text.