Resolve external embed previews and keep their real MIME types

docs/media-scraper.md

@@ -120,9 +120,13 @@ Previews run after every walk:
 - In both modes, embed rows also get their stored `preview_url` (YouTube, X, TikTok thumbnails,
   and so on) cached. The embed's full media URL is never fetched; embed rows are excluded from
   `pendingDownloads`.
+- Embeds without a thumbnail in Discord's data (fxtwitter, vxReddit, ...) are resolved first:
+  provider oEmbed/fxtwitter API, then the page's `og:image`/`twitter:image` (512 KiB read cap,
+  10 s timeout, one attempt).
 - One attempt per row, no retries; a failure marks `preview_error` and never fails the scrape.
-- Stored by hash in `thumbs/<sha256>.webp`; `preview_hash` and `preview_bytes` land on the row, and
-  the counter is `previewed`. Previews count toward the storage limit.
+- Stored by hash in `thumbs/<sha256>.<ext>`, where the extension follows the response type
+  (`preview_ext`, default webp); `preview_hash` and `preview_bytes` land on the row, and the
+  counter is `previewed`. Previews count toward the storage limit.
 - The job stays `running` through the download and preview passes, so a client never sees `done`
   on a half-archived scrape.