Improve README

The README is useful but a bit short for new users. Please add a few practical examples and a short deploy/troubleshooting section so people can get started faster.

What to do (small tasks)

Add 1 curl example showing how to upload a CSV file to POST /process/csv.
Add 1 short Python example showing how to call the JSON mode (requests + JSON body).
Add a short “Deploy on Render” subsection with the start command and required env vars (e.g. CSVAPI_EXPORT_DIR, CSVAPI_MAX_UPLOAD_MB).
Add a short Troubleshooting section with 3 tips: “No UI found”, “413 upload too large”, and “export directory/mount issues”.
Files to edit

[README.md](vscode-file://vscode-app/c:/Users/dcmau/AppData/Local/Programs/Microsoft%20VS%20Code/resources/app/out/vs/code/electron-browser/workbench/workbench.html)
Acceptance criteria

README contains the curl example and Python example.
README has a Deploy on Render section with the start command: python -m uvicorn main:app --host 0.0.0.0 --port $PORT --workers 2.
README has a Troubleshooting section covering the three tips above.