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.

VERSION

@@ -1 +1 @@
-0.28.0
+0.28.1