Scroll depth: update site.scroll_depth_visible_at in ingestion

[macobo]

We decide what metrics to show dynamically based on whether site has sent us scroll depth data.

Previously, this data was queried dynamically from ClickHouse on page loads, but this approach adds 50-200ms to every page load until a site has sent us scroll depth data.

Instead, we now update site.scroll_depth_visible_at in ingestion. To limit impact on ingestion:

1. scroll_depth_visible_at is now cached along with other ingestion site data.
2. updates are processed using ets, making sure only one database update is ever done per site
3. handles errors gracefully if postgres is ever in e.g. read-only mode or otherwise unavailable.

I'm curious whether I'm taking a sane approach with the GenServer here. cc @zoldar for feedback on that front.

[zoldar]

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