feat: document and comprehensively test download endpoints (#5)

[XananasX7]

Summary

Implements the requirements from #5 — the /files/download endpoint.

The download endpoints already existed in the codebase; this PR:

• Documents both download forms in the README
• Updates the API index page to surface the download endpoints
• Adds a comprehensive dedicated test file with 8 tests covering all edge cases

Download Endpoints Implemented

Query parameter form

GET /files/download?file_path=/path/to/file.txt
GET /files/download?file_id=<uuid>

Path URL form

GET /files/download/path/to/file.txt

Both endpoints return files with:

• Content-Disposition: attachment; filename="..." for browser download prompts
• Content-Type: text/plain for text files
• Content-Type: application/octet-stream + Content-Length for binary files
• 404 when file is not found

Tests Added

tests/routes/files-download.test.ts with 8 tests:

• ✅ Text file download via ?file_path=
• ✅ Text file download via ?file_id=
• ✅ Binary file download via ?file_path= (byte-level verification)
• ✅ 404 for missing file (query param)
• ✅ Text file download via path form (nested path)
• ✅ Binary file download via path form (byte-level verification)
• ✅ 404 for missing file (path form)
• ✅ Single-segment top-level file via path form

Validation

bun test
37 pass, 0 fail

/claim #5