[Task]: Update collection counts, tags, and dates in background job

[tw4l]

Description

When collections are created or modified (e.g. items added/removed), we currently block on recalculating collection data such as counts, tags, and date ranges. As collection sizes grow to many hundreds or thousands of items, this can make some of the API responses quite slow.

We should move these updates to either a FastAPI background task or a proper Browsertrix background job so that they don't slow down the backend.

Context

No response