[plc-replica] Expose current cursor in `_health` response

[ds84182]

Right now the cursor has to be scraped from the log lines from plc-replica.

I plan on writing my own "microservice" (in my monolithic server binary :) for synchronous handle and service lookup.

The initial backfill is done by plc-replica, then the database is exported as JSON lines (going to file an issue for this), and then it continues by using the plc-replica cursor with /export/stream. I'll then continue running plc-replica as a source to recover from if https://plc.directory/export/stream returns OutdatedCursor to my application.