[Postgres Storage] Parameter index compacting

[rkistner]

This ports #759 to Postgres storage. Postgres storage had no parameter index compacting before, so this adds a new implementation matching the new MongoDB one.

To a large extent this is a direct port of the MongoDB implementation: Postgres uses the same basic storage structure for this as MongoDB v1 storage. This does make the implementation slightly simpler, as we don't have to cater for multiple collections like MongoDB v3 storage.

There are still some notable differences due to how the parameters are read with Postgres:

1. Our Postgres storage does not attempt to detect changes to bucket data or parameter indexes yet - it refreshes them all on every new checkpoint. This removes that need for the compact fence we have for MongoDB.
2. Our Postgres storage does not use snapshot reads for parameter indexes like our MongoDB storage. It was not previously required due to not having compacting. Postgres does have some options for that, but it is more complex than on MongoDB, and can block other operations such as VACUUM on the db. Instead, we use the same compact fence concept we have on MongoDB, but use that to invalidate the entire checkpoint.

So overall we do still use the same compact fence implementation, but for a different reason.

Migration

This adds two new fields on the sync_rules table, which is a fast migration. This does not touch the potentially large bucket_parameters table.

Status & AI Usage

This is completely ported/vibe-coded using Claude Opus 5.

I have not manually reviewed this yet. The priority right now is to get #759 shipped, but in the meantime this serves to demonstrate we can quickly port the changes afterwards. For this reason I'm not using a pull request stack here.

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