Improve /lighthouse/database/reconstruct API: return informative error if backfill not enabled

[VolodymyrBg]

Issue Addressed

This PR addresses issue #4788: State reconstruction API doesn't work if genesis backfill hasn't been enabled

Proposed Changes

• The /lighthouse/database/reconstruct API endpoint now returns a clear, user-friendly error if the node was started without historic block backfill (i.e., without --genesis-backfill or --reconstruct-historic-states).
• The error message includes instructions to restart the Beacon Node with --reconstruct-historic-states.
• The API handler was refactored to propagate errors from the reconstruction logic to the HTTP response, instead of only logging them.
• Documentation (book/src/api-lighthouse.md) was updated to describe the new API behavior and provide example responses.
  Internal logic in process_reconstruction was updated to return a Result, enabling error propagation to the API layer.

Additional Info

• This change improves UX for users running archival nodes or researchers, making it clear why state reconstruction may fail and how to resolve it.
• No changes were made to the actual reconstruction logic; only error handling and documentation were improved.
• Future consideration: If dynamic enabling of backfill becomes possible, the API could be extended to trigger it directly.
• All changes are backward compatible for nodes already running with the correct flags.

[michaelsproul]

Hi @VolodymyrBg can you please re-target your PR at the unstable branch? We use unstable for all development