Update collection stats (e.g. date ranges, counts, tags) in background job

[tw4l]

Fixes #3218

This PR moves updating of collections after changes (e.g. items being added or removed) to a background job, to ensure that collection API requests remain quick.

Changes

• New background job added to recalculate collection stats
• The method to recalculate collection stats is tweaked slightly to make it more efficient, and to fix the most common tag count calculation
• All instances where collection statistics would be re-created as part of an API method now kick off a background job instead of awaiting (there are a long-running processes such as org import where we still await instead)
• Backend and nightly tests updated to account for the changes
• Frontend collection detail now polls every 10 seconds to pick up updates after changes that kick off stats recalculation (e.g. adding or removing items) - this is a fairly naive/simple implementation but seems to be working pretty well in testing
• A few places in the backend modules these changes touch where we were using asyncio.create_task have been updated so that they will not be garbage collected before they complete (see [Task]: Ensure asyncio tasks aren't garbage collected before they complete #3240 for more context and tracking of completing this across the rest of the backend)

Testing

• Spin up a Browsertrix instance
• Create a collection with some items
• Add and remove items and then verify that the collection stats update not long after
• Verify that background jobs have been created and marked as successful in the database via API

Nightly test run: https://github.com/webrecorder/browsertrix/actions/runs/23769472591