fix(backfill): include recent-merged segment in repo sync-status rollup

[philluiz2323]

Closes #377.

Summary

refreshRepoSyncStateFromSegments derived the repo-level sync status from a segment list that omitted recent_merged_pull_requests (it was loaded and used for counts, but excluded from the waiting/running/errored/incomplete checks). So when the merged-history crawl was still running / waiting_rate_limit / error while the other segments completed, the rollup produced status: "success" with a fresh lastCompletedAt. The freshness check then skips the repo for the freshness window, leaving recent-merged PR history silently stale/undercounted (and status: "success" could coexist with a stale mergedPullRequestsSyncedAt). The monolithic path's summarizeSegments already includes recent-merged, so the two paths disagreed.

Scope

• src/github/backfill.ts — include recent_merged_pull_requests in the status/warnings rollup, and add an isTerminalSegmentStatus helper that treats complete / not_modified / sampled as terminal. sampled is the recent-merged progressive-history terminal state (no other segment produces it, already treated as synced for mergedPullRequestsSyncedAt), so a sampled crawl still counts as success rather than perpetually partial.
• test/unit/backfill.test.ts — rollup test for an unfinished merged-history crawl.

No schema, API, or public-surface changes. Rebased on top of #353.

Validation

npx vitest run test/unit/backfill.test.ts                                   # 54/54
npx vitest run test/unit/queue.test.ts test/unit/data-spine.test.ts \
  test/unit/data-quality.test.ts test/integration/api.test.ts               # 92/92
npx tsc --noEmit                                                            # clean
git diff --check                                                            # clean (no whitespace errors)

Branch coverage stays >= 97%. CI validate is green.

Safety

• Pure status-derivation change: it only makes an already-tracked segment's state affect the repo status/warnings. No new external calls, tokens, or data surface.
• Strictly more conservative — it can only move a repo out of success (to running/rate_limited/error/partial) when the merged-history crawl is unfinished; a sampled/complete crawl still yields success, so it cannot wrongly force healthy repos into perpetual re-backfill.

Notes

• Distinct from fix(backfill): hydrate merged PR changed files in segment backfill #353 (recent-merged changed files) and [Bug]: buildRepoOutcomePatterns ignores the recent_merged_pull_requests table — merge-rate analysis runs almost without merged PRs, falsely flagging healthy repos as "high closure risk" #312 (the analysis ignoring recent-merged): this fixes the sync-status/freshness mechanism that was prematurely marking the crawl complete.

[JSONbored]

@philluiz2323 this is good to land after GitHub refreshes the branch state.

A few notes:

• Including the recent-merged segment fixes the status rollup without broadening the backfill scope.
• The terminal-status helper keeps the behavior understandable.
• No code changes requested.

Validation expected:

• Keep the current green CI run after the branch is updated if GitHub requires it.