backfill: use constants for bufferedOp.kind

[mrd0ll4r]

(Part of #638)

This utilizes constants from the repomgr package for buffered operations.
This was already the case, actually, as they were used here, I just added them throughout the code.

They're also used to check the operations emitted by the firehose: https://github.com/bluesky-social/indigo/blob/main/backfill/backfill.go#L469-L471
The firehose, in turn, uses the events emitted by the repo manager for the Action field in firehose events: https://github.com/bluesky-social/indigo/blob/main/indexer/indexer.go#L83-L94
This brings us full circle:
Repo manager emits diff operations -> firehose repo commit ops .Action -> backfill buffered op .kind

Notably, the constants are also given in the lexicon: https://github.com/bluesky-social/atproto/blob/main/lexicons/com/atproto/sync/subscribeRepos.json#L178
But lexgen does not generate constants for them.

The test in backfill_test.go is broken, but I changed the constants anyway.

[ericvolp12]

LGTM, thanks for the contribution!