Fix backfill for subcollections

[guenth39]

Change Summary

With these changes, the backfill will now work also for subcollections, not only for "root" collection. If the collection path is something like users/{userId}/items the sync has already worked, but the backfill did not as the documents were queried with this collection path, but that did not work. Instead, we use the collectionGroup feature now with only the last part of the path.

Closes #17

PR Checklist

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[guenth39]

Closed in favor of #52 to prevent mixing up different changes.