fix(upload): editing a caption after release now regenerates the keepsake

edit_upload updated the caption/hashtags and nothing else. A caption lives in the HTML
viewer keepsake (the ZIP holds media only — export.rs), so after release the downloadable
viewer kept showing the OLD caption forever while the live feed showed the new one.

Editing stays allowed while locked/released — like comments and likes, the lock freezes
new uploads only (USER_JOURNEYS §9.3) — so the fix is to regenerate, not forbid: the
edit and an invalidate_and_arm(ViewerOnly) share one transaction (same atomicity as
delete_upload), then start_regen after commit. ViewerOnly carries the finished ZIP forward
untouched since the media didn't change; when the gallery isn't released, invalidate_and_arm
returns None and this is a no-op. Mutation-verified: without it, an edit doesn't bump the
epoch and the caption test fails.

Also (test isolation): the compression worker now carries a generation counter that
TRUNCATE bumps. A worker queued on the concurrency semaphore when a truncate wiped media
would otherwise wake in the NEXT test, fail to find its file, and broadcast
upload-error/upload-deleted into that test's SSE stream — corrupting any test asserting on
toasts or feed. It now abandons itself when the generation moved. No-op in production
(TRUNCATE is the only caller). Export workers were already inert across a truncate (epoch
guard on a fresh-UUID event).

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

backend/src/handlers/test_admin.rs

@@ -98,6 +98,13 @@ pub async fn truncate_all(
     // surviving ticket is a dangling reference to a user that no longer exists.
     state.sse_tickets.clear();
 
+    // Invalidate any in-flight/queued compression task spawned by the previous test. Without this a
+    // task still waiting on the concurrency semaphore wakes AFTER this wipe, fails to find its
+    // (now-deleted) file, and broadcasts upload-error/upload-deleted into the NEXT test's SSE
+    // stream. (Export workers are already inert across a truncate: they are epoch-guarded on the
+    // event row, and truncate gives the event a fresh random UUID, so their writes match nothing.)
+    state.compression.bump_generation();
+
     Ok(StatusCode::NO_CONTENT)
 }