Fix COCO export timeouts with temp file approach

[JasonWildMe]

Summary

• Switches COCO export from direct-to-response streaming to a temp file model, preventing NGINX proxy timeout corruption on large exports
• Writes ZIP to uploadTmpDir first, then streams the completed file with an accurate Content-Length header
• Adds response.isCommitted() guard so error handling doesn't mix HTML into a partially-written ZIP stream
• Fixes resource leaks in EncounterCOCOExportFile: HttpURLConnection disconnect in detectContentType(), Graphics2D disposal in convertAndWriteImage()
• Ensures manifest consistency: images written first, JSON manifest references only successfully exported images

Root cause

The frontend fetch() call receives a TypeError: Failed to fetch when NGINX's proxy_read_timeout (default 60s) kills the connection while Tomcat is still streaming images into the ZIP. Tomcat doesn't detect the disconnect and logs success. The temp file decouples the slow image-fetching phase from the fast file-streaming phase.

Test plan

• Export COCO on a dataset with 2000+ images — verify ZIP downloads completely without "Failed to fetch"
• Verify temp file is cleaned up after download (check uploadTmpDir in container)
• Confirm small exports (< 50 images) still work as before
• Unit tests pass (EncounterCOCOExportFileTest)

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 11.82796% with 82 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.87%. Comparing base (55a2dc8) to head (152f6e3).

Files with missing lines	Patch %	Lines
...d/src/pages/SearchPages/components/ExportModal.jsx	11.82%	82 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1529      +/-   ##
==========================================
- Coverage   53.25%   52.87%   -0.39%     
==========================================
  Files         274      274              
  Lines        9818     9910      +92     
  Branches     2906     2931      +25     
==========================================
+ Hits         5229     5240      +11     
- Misses       4345     4426      +81     
  Partials      244      244              

Flag	Coverage Δ
backend	52.87% <11.82%> (-0.39%)	⬇️
frontend	52.87% <11.82%> (-0.39%)	⬇️

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[goddesswarship]

@naknomum bumping for your review!