Add /reference-context chunked upload endpoint

[IamJasonBian]

Summary

• New /api/reference-context endpoint for uploading large files (e.g. 50MB zips) via chunked PUT requests
• Supports single-shot uploads for small files and chunked uploads with chunk/total params for large files
• GET endpoint lists files, returns metadata with chunk completion status, or streams raw file/chunk bytes

Test plan

• Deploy to Netlify and verify /docs shows the new Reference tag
• Upload a small file via single-shot PUT
• Split a large file into chunks, upload each, verify completion status via GET
• Retrieve individual chunks and full file listing

[netlify]

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