Add Discord-hosted preview caching (C11)

docs/media.md

@@ -8,8 +8,8 @@ the byte downloader are implemented (C3-C8, C10). The Media and Bot tabs are C6.
 - slopbot on a larger channel: 11 pages, 1088 messages (96 images, 1 GIF, 203 videos), 0 errors.
 - puppet on a group DM and a 1:1 DM: done, 0 errors, backend `puppet`.
 
-Remaining: preview caching (C11) and a 429 observation on a very large walk. Checkpoints live in
-[CURRENT.md](../CURRENT.md).
+Preview caching is implemented and live-verified (2 WebP thumbs served over the thumb route).
+Remaining: a 429 observation on a very large walk. Checkpoints live in [CURRENT.md](../CURRENT.md).
 
 The media archive saves the media history of a channel. A trusted user runs `scrape media` in the
 channel. The daemon walks the history through a scrape backend and indexes every image, GIF,
@@ -80,8 +80,9 @@ media-scrape/db/
 - `download` fetches attachment bytes into `media/`, one file at a time, hash-addressed. Embed rows
   are never download targets. The Media tab prefers the local file when one exists, so the archive
   survives CDN URL expiry.
-- Cached image previews are still planned (C11). Until then, the Media tab hotlinks Discord CDN URLs
-  in `links` mode.
+- Discord-hosted image and GIF previews are cached small (WebP, 320px) in `thumbs/` after every
+  walk, in both modes, so the grid does not depend on expiring CDN links. Third-party embed
+  thumbnails are never fetched.
 - Third-party embeds are **never downloaded**: YouTube, Instagram, X, TikTok, and every other
   embed stays a link with metadata.
 - Discord-hosted image and sticker previews are cached small (`thumbs/`, WebP, via the CDN