Expose chainwork in block API responses

Bitcoin Core exposes the cumulative chainwork for each block via the getblock RPC. This value is the canonical measure used to compare competing chains and determine the active chain.

It would be useful if Esplora included a chainwork field in /block/:hash responses. This would allow clients to compare competing branches, analyze reorgs, and display cumulative proof of work without having to recompute it from every block's bits value.

Since chainwork is already maintained by Bitcoin Core, this would be an additive, backward-compatible API enhancement.

Trace the implementation of the /block/:hash endpoint and how its block data is obtained from Bitcoin Core. Confirm where the existing response is assembled and how chainwork is represented by the upstream getblock RPC. Done means block responses include a chainwork field while retaining the current response fields and behavior.

Written by the indexing model from the issue text.