fix(plex): image proxy no longer exhausts the DB connection pool

A collections page fires 100+ concurrent poster requests. Both image proxies
(/image and /person-image) authenticated via current_user + Depends(get_db),
holding a pooled DB connection for the whole request — including the slow Plex/
CDN fetch. Under the burst, the 15-connection pool was exhausted → QueuePool
checkout timeouts → 502s → slow, partially-loaded grids on prod.

Fix: authenticate these two high-fan-out endpoints with a signed-session check
(no DB user-load), serve disk-cache hits with zero DB access, and on a cold miss
open a short session only to resolve the image key + Plex config, releasing it
BEFORE the fetch (image_bytes uses no DB). Also raise the pool (20 + 30 overflow)
as headroom above the sync-endpoint threadpool.

frontend/src/lib/releaseNotes.ts

@@ -14,6 +14,14 @@ export interface ReleaseEntry {
 }
 
 export const RELEASE_NOTES: ReleaseEntry[] = [
+  {
+    version: "0.28.1",
+    date: "2026-07-06",
+    summary: "Fix: slow / broken Plex poster images on collection-heavy pages.",
+    features: [
+      "Plex images: fixed the poster/art proxy exhausting the database connection pool when a page fired many poster requests at once (the culprit behind slow, partially-loaded collection grids) — image requests no longer hold a database connection while fetching from Plex, and repeat views are served straight from the disk cache.",
+    ],
+  },
   {
     version: "0.28.0",
     date: "2026-07-06",