fix(plex): every small Plex picture asks for its card-size cut

The playlist rows, the search box and the collection picker still
fetched the 400px poster for a 32-80px picture. A contract test pins the
set of bare Plex pictures, and the srcset widths to imgcache.SIZES.

backend/app/plex/imgcache.py

@@ -27,7 +27,8 @@ _CT_EXT = {ct: ext for ext, ct in _EXT_CT}
 # page's ≤224px poster at 2× DPR, the full art (1280px) a hero backdrop. A card is much smaller: a grid
 # poster is ~144px and a shelf's landscape art ~212px, so the `_sm` pair serves those at 1.33× DPR
 # (E31 S4e: cards fetched the 400 poster, 42 KB, and the 1280 art for a 212px picture). The browser
-# picks between a pair itself (`srcset`, the frontend's `plexSrcSet`).
+# picks between a pair itself (`srcset`, built by the frontend's `lib/plexImage` cardImage, whose
+# widths plexImage.contract.test holds to these).
 SIZES = {
     "thumb": (400, 600),
     "thumb_sm": (240, 360),