improvement(plex): serve right-sized poster/art images via Plex transcode

Posters/art were proxied at full resolution (many 1-7MB, some 9.5MB) into ≤176px
grid/info cells — heavy to fetch + decode, so fast scrolling lagged 1-2s even
with a warm cache (the cache removed the Plex round-trip, not the image weight).

Now PlexClient.image_bytes optionally hits Plex's /photo/:/transcode to resize
server-side; the /image endpoint requests thumb=400x600, art=1280x720 and keys
the cache by width (old full-size files orphaned, re-cached small). Measured:
a poster 5.5MB->50KB, art 8.1MB->137KB, at 400x600 (crisp on 2x DPR). Grid
scroll is near-instant and the disk cache shrinks ~8x.

backend/app/plex/client.py

@@ -6,6 +6,7 @@ metadata, image proxying, and (P5) optional watch-state write-back. Requests JSO
 ``Accept: application/json`` header (Plex serves XML by default).
 """
 import logging
+from urllib.parse import quote
 
 import httpx
 from sqlalchemy.orm import Session
@@ -112,11 +113,24 @@ class PlexClient:
         items = mc.get("Metadata", []) or []
         return items[0] if items else None
 
-    def image_bytes(self, image_path: str) -> tuple[bytes, str]:
+    def image_bytes(
+        self, image_path: str, width: int | None = None, height: int | None = None
+    ) -> tuple[bytes, str]:
         """Raw bytes + content-type of a Plex thumb/art image (proxied to the browser). The
-        image_path is a Plex path like ``/library/metadata/123/thumb/456``."""
+        image_path is a Plex path like ``/library/metadata/123/thumb/456``.
+
+        With width+height, Plex's photo transcoder resizes server-side so the browser fetches a small
+        image (~tens of KB) instead of the multi-MB full-res original — far faster grid scroll + a
+        much smaller disk cache. ``minSize=1`` fits within the box; ``upscale=0`` never enlarges."""
+        if width and height:
+            path = (
+                f"/photo/:/transcode?width={width}&height={height}"
+                f"&minSize=1&upscale=0&url={quote(image_path, safe='')}"
+            )
+        else:
+            path = image_path
         try:
-            r = self._http.get(f"{self.base}{image_path}")
+            r = self._http.get(f"{self.base}{path}")
             r.raise_for_status()
         except httpx.HTTPError as e:
             raise PlexError(str(e)) from e