fix(download): dispatch archive/non-media in attach_in_place; reshuffle showcase on mount

attach_in_place mirrored only the media flow, so gallery-dl-downloaded
zips/PDFs/audio bounced back as `skipped+invalid_image`, which
download_service counted as an ingest error and flipped runs to
status="error" despite N successful image attaches. Lustria patreon
event #38998 (21 images + 1 OST zip) went red for exactly this reason.

Now attach_in_place dispatches the same way as import_one: archives →
_import_archive (extracts media members, captures archive as
PostAttachment), non-media → _capture_attachment. Download_service
accepts the new `attached` result and treats non-duplicate skips as
soft skips, not ingest errors.

Also: ShowcaseView always loadInitial() on mount, not just when the
store is empty — Pinia persists across navigations and operator wants
a fresh shuffle every time the showcase loads.

frontend/src/views/ShowcaseView.vue

@@ -70,7 +70,12 @@ function onKeydown(e) {
 }
 
 onMounted(() => {
-  if (store.images.length === 0) store.loadInitial()
+  // Operator-stated 2026-06-02: every load of the showcase view should
+  // show new images. Pinia persists the store across navigations, so
+  // returning to /showcase used to render the same set as before. Now
+  // we always reshuffle on mount — loadInitial() resets `seen`,
+  // `images`, and `exhausted` and pipelines a fresh batch.
+  store.loadInitial()
   window.addEventListener('keydown', onKeydown)
 })
 onUnmounted(() => window.removeEventListener('keydown', onKeydown))