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perf(asset): store version content hash in DB to avoid loading its content DEV-1841 (#6843)

### 📣 Summary

Opening a project with many deployed versions and many questions could
return a 502 error.

### 👷 Description for instance maintainers

In the API response of the asset detail endpoint
(`/api/v2/assets/{asset_uid}/`), the hash of the content of each version
was computed on the fly, loading all their JSON into memory (could be
many MB, even GB of objects in memory at once).

A new database field now stores the hash directly on each version
record, computed once when the version is saved. A long-running
migration backfills the hash for all existing records in small batches
to avoid memory spikes.

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