perf(web): stream files in 4 MiB chunks + 4 hypercorn workers (fix 40s downloads)

The image library is on a CIFS/SMB share (mounted rsize=4 MiB, actimeo=1), and
Quart's FileBody streams in 8 KiB chunks — so serving one large original was
~19k network round-trips to the storage server, i.e. 30–58s per download
(operator-flagged). That's what starved the GPU agent (constant "curator
unreachable" backoff) AND slowed the browser: every byte is read off CIFS and
streamed through the Python app (no reverse-proxy sendfile), and only 2 hypercorn
workers meant the agent + the browser's thumbnail grid queued behind each other.

In-container fix, no new service:
- Raise FileBody.buffer_size 8 KiB → 4 MiB in create_app, matching the mount's
  read size: one round-trip per read, ~500× fewer. buffer_size is the MAX read so
  small thumbnails still read in one gulp, and Range/mime/ETag/conditional
  handling lives on Response — all preserved. Guarded so a Quart-internal change
  can't break boot.
- HYPERCORN_WORKERS default 2 → 4 so concurrent /images requests stop queuing.

Expected: large-file transfers drop from ~40s toward link speed (a few seconds)
for the agent and the browser. See issue #1223.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

entrypoint.sh

@@ -12,9 +12,13 @@ case "$ROLE" in
     # create_app is a factory — the `()` tells hypercorn to call it once
     # and serve the returned Quart (ASGI) app, rather than treating the
     # function itself as the application (which it then mis-invokes as WSGI).
+    # Default 4 workers (was 2): each worker is one asyncio loop, and a large
+    # file download occupies its worker for the transfer — 2 was too few once the
+    # GPU agent + the browser's thumbnail grid hit /images concurrently (they
+    # queued behind each other). Env-tunable via HYPERCORN_WORKERS.
     exec hypercorn \
       --bind 0.0.0.0:8080 \
-      --workers "${HYPERCORN_WORKERS:-2}" \
+      --workers "${HYPERCORN_WORKERS:-4}" \
       --access-logfile - \
       "backend.app:create_app()"
     ;;