Add video poster frames and rework the media status row

docs/media.md

@@ -138,12 +138,14 @@ that deletion is irreversible. The delete button is disabled for a 5 second coun
   21:9, Other), resolution (SD, HD, FHD, QHD, 4K+, Unknown), and a posted-date range (from/to).
   Nothing renders until **Load media** is pressed. A **Metadata** dropdown toggles which fields
   show in the small banner under each tile (User, Type, Date, Source, Resolution, Length). Video
-  tiles carry a translucent play icon top-right. Clicking a tile opens a viewer: wide media shows
-  the metadata banner below, vertical media shows it to the right; the banner has the full link
-  (with an external-site warning) and **View full**, and the play action embeds a YouTube, Vimeo,
-  Twitch, or Streamable player when possible. The status row reads
-  `downloaded A/B (images/videos) | cached C/D (excludes downloaded) | N loaded`. A scrape strip
-  shows live progress with Stop.
+  tiles carry a translucent play icon top-right. Discord serves no thumbnails for uploaded videos,
+  so a video without a cached preview renders a first-frame `<video>` in the grid, and downloaded
+  videos get an ffmpeg poster cached in `thumbs/`. Clicking a tile opens a viewer: wide media
+  shows the metadata banner below, vertical media shows it to the right; the banner has the full
+  link (with an external-site warning) and **View full**, and the play action embeds a YouTube,
+  Vimeo, Twitch, or Streamable player when possible. The status row reads
+  `(Images/Videos) | Downloaded A/B | Cached C/B (excl Downl) | Loaded L/B`, where B is the total
+  for the active filters. A scrape strip shows live progress with Stop.
   **New scrape…** (rightmost) opens a popup with Server and Group chat trees (servers expand to
   channels; each row has Links and Download). Every scrape button asks for confirmation; the
   dialog takes optional From/To dates and a **Check Message Count** button (Discord search total,