## Summary
Closes#1546. `/api/stats` reported
`{"backfilling":true,"backfillProgress":0}` on every fully-converged
server, and `X-CoreScope-Status: backfilling` was sent on every request.
Root cause: the `Store` had three atomic fields — `backfillComplete` /
`backfillTotal` / `backfillProcessed` — read by `handleStats` and
`backfillStatusMiddleware`, but **nothing ever wrote to them**. They are
leftovers from the server-side async backfill added in #612/#614. That
work moved to the **ingestor** in #1289 (server is now read-only) and
the writer `backfillResolvedPathsAsync` was deleted, orphaning the
readers. `backfillComplete.Load()` therefore always returned `false`, so
`backfilling := !false` was permanently `true`.
This is the leftover of an intentional architecture change, not an
unfinished feature — the server no longer does backfill by design, so
the correct fix is to delete the dead flag (per triage recommendation;
zero consumers).
## Changes
- `store.go` — drop the 3 dead atomic fields.
- `routes.go` — drop `backfillStatusMiddleware` (+ its registration) and
the backfill-progress computation in `handleStats`.
- `types.go` — drop `Backfilling` / `BackfillProgress` from
`StatsResponse`. **API change:** `/api/stats` no longer emits
`backfilling` / `backfillProgress`; the `X-CoreScope-Status` header is
removed. Verified no frontend or other consumer reads them.
- `resolved_index.go` — remove stale comment referencing the deleted
`backfillResolvedPathsAsync`.
## Test
Regression assertion added to `TestStatsEndpoint` (#1546): asserts the
response no longer carries `backfilling` / `backfillProgress` and that
`X-CoreScope-Status` is unset. Verified red→green — against pre-fix code
all three assertions fail; with the fix they pass. Full `cmd/server`
suite green locally.
## Out of scope
If a real server-side backfill/migration status indicator is wanted,
that's a new feature on top of the ingestor stats pipe — tracked
separately, not by reviving these dead fields.
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
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