Alert on proving progress failures or stalls

If generating proofs fails or stalls when generating a proof we should be alerted. For example if `OPSuccinctDataFetcher` fails when fetching boot info. This is probably regular metrics reporting from the proposer and a grafana rule that alerts if the metrics aren't reported.

Start at OPSuccinctDataFetcher and trace the proposer’s proof-generation and boot-info-fetching paths. Identify the existing metrics reporting and Grafana configuration, then define an alert for missing or failed progress metrics. Done means proof-generation failures or stalls produce an actionable alert.

Written by the indexing model from the issue text.