fix(rereview): close residual export finalize↔flip double-race at reopen

Adversarial re-review of df275bb found the two-guard ready-flip fix still left a
narrow double-race open, plus test-hygiene drift from the banUser param removal.

MED — residual stale-keepsake race the `export_released_at` guard alone missed
  The flip relied on two guards defeating two clearers: the `release_seq` EXISTS
  check (vs a re-release bump) and `export_released_at IS NOT NULL` (vs open_event's
  clear). But release_gallery re-arms `export_released_at = NOW()` and bumps
  `release_seq` as SEPARATE statements, so a stale worker's flip landing in the gap
  between them satisfies BOTH guards (released re-armed, seq not yet bumped) and
  resurrects a pre-reopen keepsake — the next re-release then skips regeneration and
  serves an archive missing the reopen-window uploads.

  Root-cause fix: `open_event` now bumps every `export_job.release_seq`, making a
  reopen a supersession point symmetric with a re-release. A worker that captured the
  pre-reopen seq can never match its `release_seq`-guarded finalize/flip again,
  regardless of when the re-release re-arms `export_released_at`. The released-anchor
  guard stays as defense-in-depth. Added a deterministic e2e asserting the reopen
  bumps the seq (the invariant that closes the race without depending on
  sub-millisecond worker timing).

LOW
  - Gate the `export-progress: 100` SSE + `prune_stale_export_files` on the ready-flip
    actually flipping (rows_affected > 0). A superseded worker no longer advertises a
    misleading 100% or prunes on a fresh generation's behalf.
  - moderation.spec.ts: the two ban tests had become byte-identical after the
    hide_uploads param removal; drop the one whose "hide_uploads=true" title no longer
    matched what it exercised, keep the accurate "always hides" test.
  - host-dashboard page-object: drop the dead `banUser(hideUploads)` param + its
    checkbox branch (the UI no longer renders that checkbox — a latent hang trap).
  - sse-eviction.spec.ts: rename the test whose title referenced the removed flag.

Verified: backend 40 tests, e2e 156 passed / 1 skipped on chromium-desktop
(incl. the new reopen-supersession regression).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/handlers/host.rs

@@ -528,6 +528,24 @@ pub async fn open_event(
     .await?;
 
     if result.rows_affected() > 0 {
+        // A reopen invalidates any released keepsake, so make it a *supersession point* for the
+        // export pipeline: bump every export_job generation for this event. An in-flight worker
+        // — or one that has already `finalize_job`'d but not yet flipped its ready flag — captured
+        // the pre-reopen `release_seq`, so its guarded finalize/ready-flip now match nothing and
+        // become no-ops. Without this, the `export_released_at IS NOT NULL` flip guard alone leaves
+        // a sub-window open: a re-release re-arms `export_released_at` *before* it bumps the seq, and
+        // a stale worker's flip landing in that gap satisfies both guards and resurrects a
+        // pre-reopen keepsake (silent data loss). Bumping here closes it — the old seq is retired
+        // for good and the next re-release regenerates from a current snapshot.
+        sqlx::query(
+            "UPDATE export_job SET release_seq = release_seq + 1
+               FROM event e
+              WHERE e.id = export_job.event_id AND e.slug = $1",
+        )
+        .bind(&state.config.event_slug)
+        .execute(&state.pool)
+        .await?;
+
         let _ = state.sse_tx.send(SseEvent::new("event-opened", "{}"));
     }