feat: add ability to download uploaded schema/asset content

[glimow]

Problem

There is no public API endpoint to retrieve the content of uploaded files (OpenAPI specs, Postman collections, GraphQL schemas). Once uploaded via POST /upload/signed-url, the content is write-only — it can be attached to profiles but never read back.

Impact

This blocks CLI workflows that need to:

• Back up or version-control profile schemas
• Convert between formats (e.g. Postman → OpenAPI) and re-upload
• Migrate schemas between profiles or environments
• Audit what schema a profile is actually using

Proposed solution

Add a GET /assets/{assetId}/content endpoint (or similar) that returns the raw content of uploaded schema files. The CLI would then expose an assets download <asset-id> command.

Workaround

Download files manually from the web UI, or obtain them from the original source.