fix: send CSV imports as JSON instead of multipart/form-data

[jeremydw]

Problem

The CMS translations CSV import (LocalizationModal) uploads the selected file to /cms/api/csv.import as multipart/form-data. Some production deployments sit behind WAFs/edge proxies that block multipart/form-data POSTs outright, so the request is rejected before it ever reaches the server and the import fails with an opaque error.

Change

• csv.import now accepts a JSON body ({"csv": "<csv text>"}) in addition to the existing multipart/form-data upload (kept for backwards compatibility).
• The CMS UI reads the selected file client-side (file.text()) and sends it as JSON.
• /cms/api/csv.import is included in the routes that use the larger (4mb) body-parser JSON limit, matching the AI prepare routes, so larger translation CSVs are not rejected by the default 100kb limit.

Notes

• CSV files are text, so file.text() + JSON encoding is lossless.
• No API-consumer breakage: existing multipart clients continue to work; the response shape is unchanged.

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