Resolve message attachment

Resolve one attachment on a message to a media url that works right now.

Instagram and Facebook sign DM media urls per request and expire them, so the url on a message is a snapshot: it works when you read the message and stops working later. This endpoint checks the stored url and, when it has gone stale, re-mints the message's media from Meta and persists it before answering. The message id never expires, so this URL is the one to store. It is returned ready-made on each attachment as refreshUrl when you read a message over REST.

Webhook payloads do not carry refreshUrl, so a webhook-driven integration builds this URL itself. Every piece is in the event: message.conversationId, message.platformMessageId, the attachment's zero-based position, and account.accountId. accountId is a required query parameter; omitting it returns 400 missing_required_field, which is the same requirement GET /v1/whatsapp/media/{mediaId} has.

By default it responds 302 to the live media url, so it can be used directly as an <img src> on a browser session. API-key integrators should pass ?format=json and read url off the body, since a browser cannot attach an Authorization header to an image request.

Only Instagram and Facebook media can be re-minted. On other platforms the stored url is returned as-is when it still resolves, and 404 otherwise.