Five stacked optimisations for the thumbnail hot path so the timeline
grid lands in fewer round trips and fewer bytes.
1. PhotoThumbnail: switch from 'medium' (640px) to 'small' (240px) for
grid cells. 240px oversamples 150-200px logical cells on 2x retina
and drops payload 5-8x. Lightbox and preview filmstrip keep 'large'
and 'medium' respectively.
2. nextcloud_dav: pool the httpx client. A module-level AsyncClient
with HTTP/2 + keepalive (max_connections=64, keepalive_expiry=120s)
replaces the per-request constructor that paid a fresh TCP+TLS
handshake on every preview fetch. Auth is per-user so it stays at
the call site via auth=BasicAuth(...). Lifespan-managed: init in
main.py's lifespan startup, aclose on shutdown. requirements.txt
gains the http2 extra to pull in h2 (not currently installed).
Same change applies to fetch_memories_info_async since it hits the
same host.
3. PhotoThumbnail img: add decoding="async" so JPEG/WebP decode moves
off the main thread, plus fetchPriority="low" so grid backfill
doesn't fight UI fetches.
4. Eager-load Photo.user via joinedload from the thumb handler.
_get_photo_with_share_fallback gains an options parameter so other
callers stay zero-overhead; only the thumb handler asks for the
owner join. Eliminates the second SELECT users per request.
5. Disk-fallback path picks up Cache-Control: private, max-age=86400
in both the FileResponse and X-Accel branches so re-renders match
the NC primary path's caching behaviour.
Net: a warm grid page should drop from ~200-400 ms median per thumb to
well under 100 ms; payload drops ~5-8x; backend sustains higher
concurrency with fewer sockets to Nextcloud and one fewer Postgres
round-trip per request.
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