freshness metrics

[leoyvens]

This adds freshness tracking for dump jobs via two new metrics:

• latest_segment_timestamp: Unix timestamp of the end block in the most recently
  written segment
• latest_segment_delay_seconds: Difference between latest_segment_timestamp and wall
  clock time at write

For derived datasets, freshness is recorded inline when a segment is written, since writes are sequential.

For raw datasets it's more involved: multiple writers dump disjoint block ranges in parallel, so recording freshness at write time would give inconsistent values depending on which writer finishes last. Instead, a background task subscribes to table change notifications and reports freshness based on synced_range.

Commits:

1. Add timestamp field to BlockRange, propagated from block data through segment
   metadata.
2. Add the two freshness metrics and record them for derived datasets (inline at write
   time).
3. Add background freshness tracking for raw datasets via notification subscription.