feat(ingest): hydrate pull requests onto the board (RIG-4034)

[rigel-mintaka]

This PR is part of a stack containing 6 PRs:

1. main
2. feat(store): store pull requests and their issue links (RIG-4034) #1900
3. feat(forge): read PR timestamps, closing refs and PR list rows (RIG-4034) #1914
4. feat(board): show linked pull requests on board issues (RIG-4034) #1917
5. feat(forge): link a created pull request to its issue (RIG-4034) #1926
6. "feat(ingest): hydrate pull requests onto the board (RIG-4034)" (this PR)
7. feat(ingest): retry PR hydrates held by a budget pause (RIG-4826) #1971

Summary

This is T5 of the PR-to-issue linkage design (docs/designs/server/compass-issue-model-pr-linkage-amendment/design.md, §4). It stacks on the T4 PR.

• Webhook arm
  • Admits every PR event.
  • Resolves a SHA-only CHECKS event through the notify lane's shared resolver cache; a disabled repo spends no lookup.
  • Keys coalescing on repo, kind, and number or SHA.
• PullRequestHydrator: reads, owner-strips, stamps and sinks a PR through PublishPullRequestUpdate.
• Reconciler
  • Hydrates listed PR rows newer than the stored forge_updated_at.
  • On cold start, skips closed PRs older than 30 days.
  • A budget error on a PR row keeps the watermark at since with no ETag.
• Backfill
  • Runs once per repo over open PRs plus rows updated in the 30 days before the watermark.
  • A failed row leaves the repo unmarked, so the next sweep retries.
• Store and cache
  • BoardStore gains PullRequestUpdatedAt, PRsBackfilledAt and MarkPRsBackfilled.
  • The head-SHA cache key ignores repo case.

Verification

• Ingest unit tests cover every case in the design's T5 test list, plus:
  • the disabled-repo lookup skip;
  • the resolver budget pause;
  • coalescing in both orders;
  • the backfill retry.
• Fixes were mutation-probed red.
• A pgtest runs the backfill through the real store adapter.
• go test ./..., pgtests and golangci-lint pass.

Risks

• PR hydrates spend the shared App budget, about one per coalesced burst.
• The backfill walks a 30-day window once per repo.

Compatibility

No wire change.

Documentation

None. The design record covers it.

Spec-impact: none. Refs RIG-4034

[linear-code]

RIG-4034

[github-actions]

Compass engineering docs preview: https://compass-obs-rig-4034-t5-inge.compass-eng-docs.pages.dev

Deployed from compass-obs/rig-4034-t5-ingest at 9f04574.