fix(e2e): stop the video poster assertion racing the ffmpeg thumbnail

Pre-existing, and it fired for real during the full-suite run on a cold stack.

The lightbox binds `poster={upload.thumbnail_url ?? undefined}`, so the attribute is
absent until compression produces the thumbnail. This test asserted on it immediately
after seeding, never waiting for the worker -- unlike the Range test further down the
same file, which does poll. Against a warm stack the worker usually wins; against a
freshly rebuilt one (`stack:down -v`, cold ffmpeg) it doesn't.

That is the worst possible time for a false failure: the first run after a rebuild is
exactly when you are trying to establish whether a change broke something. Poll for
`compression_status = 'done'` before the poster assertion. The `src` assertion needs
no wait and keeps none.

Verified with --repeat-each=3.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

e2e/specs/03-feed/video-playback.spec.ts

@@ -40,10 +40,19 @@ test.describe('Video — the lightbox plays it', () => {
     page,
     guest,
     signIn,
+    db,
   }) => {
     const g = await guest('VideoWatcher');
     const id = await seedVideo(g.jwt);
 
+    // The poster assertion below needs the ffmpeg thumbnail to EXIST — the lightbox binds
+    // `poster={upload.thumbnail_url ?? undefined}`, so the attribute is simply absent until
+    // compression finishes. Without this wait the test races the worker and fails against a
+    // cold stack (first run after `stack:down -v`, cold ffmpeg), which is exactly when a suite
+    // is least likely to be believed. The `src` assertion is unconditional; only the poster
+    // needs the wait.
+    await expect.poll(() => db.compressionStatus(id), { timeout: 60_000 }).toBe('done');
+
     await signIn(page, g);
     await page.goto('/feed');