Upload Echo Attachment

Upload one or more media files (image/PDF) for use in an Echo chat message.

Validates the request origin against the workspace's allowed domains, enforces a 10MB per-file limit, and persists each file to Spaces. Returns a list of attachment descriptors (fileName, fileUrl, size, mimeType, ...) ready to be embedded in the next POST /api/public/echo/message call as attachments.