fix(images): background jobs skip files hidden by the removal mark; walkers skip trash folders inside output roots (#501)

- Thumbnail jobs and the legacy scan_output_dir listing leave
  library-removed files out (one mark check, mark_hides).
- Missing-file links also forgets removal marks that hide nothing any
  more (file gone from a folder that exists, or changed), only under a
  readable, non-empty source, re-checked at apply.
- The sidebar Images, Favorites and Needs review counts and the
  collection counts are read again when an image_delete job ends.
- utils/media_folders.py: one skip for thumbnails and trash folders
  (.Trash, .Trash-<uid>, .Trashes, $RECYCLE.BIN) used by every walker,
  the image monitor and the delete review.

tests/browser/workspace-images-delete.spec.mjs

@@ -73,6 +73,19 @@ for (const mode of ['library', 'trash']) {
     });
 }
 
+// #501: the sidebar's Images count (read once at mount) follows a finished delete without a reload.
+test('a finished delete re-reads the sidebar Images count', async ({ page }) => {
+    await openImages(page);
+    const badge = page.locator('#primary-nav a[data-route="images"] .nav-count');
+    await expect(badge).toHaveText(/^\d[\d,]*$/);
+    const before = Number((await badge.textContent()).replace(/,/g, ''));
+    await check(page, 0);
+    const { primary } = await deleteChecked(page, 'library');
+    await primary.click();
+    await expect(toast(page)).toContainText('Removed 1 image from the library.');
+    await expect(badge).toHaveText((before - 1).toLocaleString('en-US'));
+});
+
 test('Cancel changes nothing', async ({ page }) => {
     const before = await openImages(page);
     await check(page, 0);