Add Discord-hosted preview caching (C11)

CURRENT.md

@@ -37,6 +37,9 @@ mobile draft at 390px.
     embeds excluded in SQL; local files served at `/api/media/<scrape>/<item>/file` and preferred
     by the Media tab. Live run 2026-09-16: scrape `...-20260916083401` fetched the 784 KB
     attachment; storage counted actual bytes.
+  - [x] C11 preview caching: Discord-hosted image/GIF previews (WebP 320px, hash-addressed, 5 MiB
+    cap, one attempt) run after every walk in both modes. Live run `...-20260916090722`:
+    `previewed: 2`; thumbs served over `/api/media/<scrape>/<item>/thumb` (200).
   - [~] C9 live verification. Slopbot path verified live on 2026-09-16: login Slopbot#5775, guild
     `177931418704281600`. Test channel links scrape: 17 messages, 4 images + 13 videos, 0 errors;
     download scrape fetched the 784 KB attachment. Large channel `gernal_chut`: 11 pages, 1088
@@ -44,8 +47,8 @@ mobile draft at 390px.
     verified live on a group DM (`zıbb & fred`) and a 1:1 DM: backend `puppet`, done, 0 errors.
     The puppet field check is done too: the group DM returned attachments plus mapped embeds
     (YouTube, Instagram), and the large slopbot channel produced YouTube, Gfycat, Imgur, Steam,
-    and Twitch embeds alongside attachments. Still owed: preview caching (C11) and a stress walk
-    on the second guild (`559346566754729995`, channel `559346566754729997`).
+    and Twitch embeds alongside attachments. Still owed: a stress walk on the second guild
+    (`559346566754729995`, channel `559346566754729997`).
 - Flextalk voice profile, end to end. The profile window is a movable panel that recenters on each
   open; it carries the character sliders with engine defaults and +/-2 snapping, five named
   presets, radio-kit effects (high-pass 40-400 Hz, echo 0-100, word mode with a gap in ms), and