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

backend/app/__init__.py

@@ -34,6 +34,23 @@ def create_app() -> Quart:
     app = Quart(__name__)
     app.secret_key = cfg.secret_key
 
+    # Stream files in 4 MiB chunks instead of Quart's 8 KiB default. The image
+    # library lives on a CIFS/SMB share (mounted rsize=4 MiB), so 8 KiB reads
+    # meant ~19k network round-trips for one large original — 30–58s downloads
+    # that starved both the GPU agent and the browser (operator-flagged
+    # 2026-07-01). 4 MiB matches the mount's read size → one round-trip per read,
+    # ~500× fewer. buffer_size is the MAX read, so small thumbnails still read in
+    # a single gulp, and Range/mime/ETag/conditional handling lives on Response,
+    # so this keeps all of it. Guarded so a future Quart-internal change can't
+    # break boot — worst case we fall back to the slow default.
+    try:
+        from quart.wrappers.response import FileBody
+        FileBody.buffer_size = 4 * 1024 * 1024
+    except Exception:
+        logging.getLogger(__name__).warning(
+            "could not raise FileBody.buffer_size — file serving stays on 8 KiB chunks"
+        )
+
     for bp in all_blueprints():
         app.register_blueprint(bp)
     # Registered last so /api/* routes win over the SPA catch-all.