fix: the poster ladder read a file already deleted

The still is the input for three things — the placeholder, poster.webp, and now the poster rungs — and
it is removed once they are done. I added the rung loop BELOW the removal, so sharp was handed a path
that no longer existed and every video failed to convert.

The comment warning that these lines are ordered deliberately was three lines above the mistake.

The removal now sits after everything that reads it, which is where it should have been all along
rather than in the middle of the sequence.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

api/derive.ts

@@ -658,17 +658,18 @@ async function buildVideo(
     order rather than the other one.
   */
   const placeholder = await encodePlaceholder(stillJpg, join(staging, 'placeholder.webp'), email);
-  await rm(stillJpg, { force: true });
   variants.push({ file: 'poster.webp', width: width || null, height: height || null, bytes: poster.bytes });
 
   /*
     The poster's own ladder, cut from the same still.
 
     From `stillJpg` rather than from `poster.webp`, so each rung is scaled once from the original frame
-    instead of being a re-encode of an encode. The still is a JPEG at the video's full size and it is
-    about to be deleted anyway.
+    instead of being a re-encode of an encode.
 
-    Rungs narrower than the source only: one at or above it would be `poster.webp` under another name.
+    EVERYTHING that reads the still happens above the line that removes it, and that ordering is the
+    only thing holding this together — the placeholder, then poster.webp, then these. This loop was
+    added below the removal on 2026-08-30 and every video failed to convert until it moved. The comment
+    warning about exactly this was already three lines up.
   */
   for (const rung of POSTER_LADDER) {
     if (!width || rung >= width) continue;
@@ -686,6 +687,10 @@ async function buildVideo(
     });
   }
 
+  // Last, once nothing else needs it. It is an intermediate: a JPEG in a directory where everything
+  // served is WebP, and leaving it would publish a rendition nothing references.
+  await rm(stillJpg, { force: true });
+
   return {
     kind: 'video',
     source: { name, bytes: source.bytes, mtimeMs: source.mtimeMs, width: width || null, height: height || null },