Option to set timezone for event snapshot timestamps

**Describe what you are trying to accomplish and why in non technical terms**
Currently, the timezone of timestamps embedded in event snapshots seems to be either UTC or the host timezone. It would be great if it were possible to set this to the actual timezone of the geographical site being surveilled.

**Describe the solution you'd like**
In Frigate 0.12, it is possible to set the timezone of the UI using the `timezone` setting in the `ui` section. One solution would be to have that control the timezone of the embedded timestamps, too. Another would be to have a separate setting. I can't think of a scenario where you would want different timezones for those settings, but perhaps one exists.

A third solution would be to have a setting for the geographical site location (coordinates), which Frigate could use to infer various things, including the timezone, but also the position of the sun/moon in the sky and whatnot that could possibly have implications on video surveillance in some future version. (Perhaps this should be a per-camera setting in case people are surveilling vast sites.)

**Describe alternatives you've considered**
Perhaps changing the host timezone would help? However, IMHO, servers should always have their timezone set to UTC regardless of physical location.

**Additional context**
Cf #5018.

Start by reading the existing UI timezone setting described in the issue and the related discussion in #5018, then trace where event snapshot timestamps are produced. Done means the embedded timestamps use the configured site timezone consistently, with the chosen configuration behavior documented and covered by the relevant existing tests.

Written by the indexing model from the issue text.