refac: atomic merge, lost-update race, RunPurge cost, duplicated walks

features/media_purge.qa.md

@@ -57,7 +57,7 @@ Package: `test` (black-box). New files `test/media_purge_http_test.go` and `test
 27. Fingerprint: match gives 200. A stale fingerprint gives 409 with `apperr.PurgeConflictErrorMessage + "\n"`, no `Delete`, no `UpdateBody`, no `Save`. A garbage string gives 409 with the same body. No fingerprint, `{}` and an empty body all run.
 28. Empty plan: 200 zeros, `Save` with `last_purge_at` = clock and zero counters, and a run with the empty plan's fingerprint is also 200.
 29. Partial failure, rolled back: arm `UpdateBody` to fail for the second article of group B (plan order A, B, C). Assert the body counters, B's bodies restored byte for byte, all B images present, A and C merged, B's articles absent from `articles_updated`, and `Save` counters equal the response.
-30. Partial failure, delete: arm `failDeleteFor` for one of group A's merged images and plan B too. Assert `failed_groups` 1, `groups_purged` 1, A contributes 0 to `merged` and none of its ids to `articles_updated` (only B counts), A's article bodies stay switched to the kept copy, and a following preview plans A again. Run the purge again with the failure disarmed and assert it converges (A's leftover copy is merged away). Then arm every `UpdateBody` and assert `groups_purged` 0, `failed_groups` = `groups_total` and `last_purge_at` still saved.
+30. Partial failure, delete: arm `failDeleteFor` for one of group A's merged images and plan B too. Assert `failed_groups` 1, `groups_purged` 1, A contributes 0 to `merged` and none of its ids to `articles_updated` (only B counts), A's article bodies are back to their original text, and a following preview plans A again. Run the purge again with the failure disarmed and assert it converges (A's leftover copy is merged away). Then arm every `UpdateBody` and assert `groups_purged` 0, `failed_groups` = `groups_total` and `last_purge_at` still saved.
 31. Concurrency: start run 1 in a goroutine, blocked in the image fake (wait on a "started" channel). Send run 2 (once with a fingerprint, once without). Assert both answer 409 with `apperr.PurgeBusyErrorMessage + "\n"` immediately (use a timeout) and no `Save`/`Delete` calls from them. Release run 1, assert its 200 body and that a later run is accepted. Run this test with `-race`.
 32. Sentinels at the use-case level, asserted separately: a stale fingerprint gives `errors.Is(err, purge.ErrPurgeConflict)` true and `errors.Is(err, purge.ErrPurgeBusy)` false; a busy guard gives `ErrPurgeBusy` true and `ErrPurgeConflict` false. Call RunPurge without any HTTP package to prove it is reusable (that test file section imports no `net/http`).
 33. Malformed body gives 400 `apperr.BodyErrorMessage`. `ListDuplicates` failure gives 500 and no `Save`.