fix: Backfill truncates PR/Issue labels, reviews, and label-event history

[jonathanchang31]

Summary

• Fixed a backfill data-loss bug where PR/Issue nested GraphQL data was truncated by fixed page sizes.
• Backfill now retrieves full datasets for:
  • PR labels
  • PR reviews
  • PR label/unlabel timeline events
  • Issue labels
  • Issue label/unlabel timeline events

Related Issue

Closes: #68

Change Type (select all)

• Bug fix
• Refactor (query/fetch flow restructuring)
• New feature
• Documentation

Real Behavior Proof

• Before:

  • PR query contained labels(first: 10), reviews(first: 10), timelineItems(first: 30).
  • Issue query contained labels(first: 10), timelineItems(first: 30).
  • Only partial nested nodes were persisted.

• After:

  • Backfill loops call pagination helpers and iterate until pageInfo.hasNextPage is false.
  • Persisted labels/reviews/timeline events now include all pages for each PR/Issue.

Impact

• Prevents undercounting in mirror data consumed by scoring/validation flows.
• Improves historical accuracy for repos with high review/label activity.
• Aligns backfill completeness with expected mirror reliability.