Track event indexes in block

[pro-wh]

Imported from https://app.clickup.com/t/8694jpmft

In both consensus and runtimes, an event can be uniquely identified by the (block height, consecutive idx) pair. We don't track the latter, but we should.

Task:

1. add event index to the DB (NOTE - requires DB reindex, hence high-ish priority)
2. expose it in the API (? - optional, possibly not yet needed)

Motivation:

This will enable us to link to single events from the activity table. We'll either deep-link into the table of a block's events, or we'll create a new FE page that shows a single event.