Presign a chat attachment upload

Step one of sending a file. Creates an attachment slot in the conversation and returns its id with an upload URL valid for 5 minutes: PUT the file's bytes to signedUrl with the Content-Type you declared as mimeType, then pass the id in attachmentIds of POST /chat/threads/{chatId}/messages (up to 10 per message). Until a message carries it, nobody sees the attachment. Images take up to 25 MB, PDFs up to 25 MB and videos up to 512 MB; at most 30 unsent uploads per conversation and hour. A sent file is served from a permanent public URL, which deleting the message does not revoke.