request a presigned URL to upload raw session transcript to S3

Generate an S3 presigned PUT URL and object key for uploading raw session transcripts directly to S3.

Upload Rules:

• Declare the exact size of the file in bytes as content_length. The maximum is 10485760 bytes (10MB), the same limit applied to curated session data.
• PUT the file to presigned_url within 15 minutes, sending exactly the returned headers. The size is part of the signature, so S3 rejects a body of any other length.
• Rate limited per user to a configurable number of URLs per configurable window (by default 20 per hour), counted whether or not a session is later created. Exceeding it returns 429 with a Retry-After header. Forem admins can change both values, or set the limit to 0 to disable uploads.