Persist open PR inventory for backfill

[dutifulbob]

Summary

• persist the fetched open-PR set in a durable inventory table instead of rescanning GitHub every backfill batch
• drive open-PR backfill from that inventory and update repo counters/cursor after each PR
• add regression coverage that proves the open-PR listing endpoint is only hit during fetch passes

Why

The backfill worker was still walking the full open-PR list before and after every batch. That made openclaw/openclaw needlessly expensive to backfill and left repo status counters lagging until the batch finished.

Validation

• go test ./internal/githubsync/...
• go test ./...
• go vet ./...
• go build ./cmd/ghreplica && go build ./cmd/ghr
• npm run docs:check

[dutifulbob]

Final report:

• Persisted a durable repo_open_pull_inventories table and wired the fetch pass to refresh it from the open-PR list.
• Switched open-PR backfill to walk that stored inventory instead of relisting all open PRs before and after every batch.
• Moved repo counters forward per PR during backfill by updating the sync-state row transactionally with each visited inventory item.
• Added regression coverage that proves multi-batch backfill only hits the open-PR listing endpoint during the fetch pass.

Validation:

• go test ./internal/githubsync/...
• go test ./...
• go vet ./...
• go build ./cmd/ghreplica && go build ./cmd/ghr
• npm run docs:check

GitHub review status:

• codex review --base main was run against this head. It traversed the diff locally but did not surface actionable findings or leave GitHub comments.
• PR issue comments: none
• PR inline review comments: none
• CI: validate passed