Get a presigned S3 URL and redirect the user's browser to download directly from S3.

This endpoint validates the file exists, generates a short-lived presigned URL (1 minute), and redirects the browser to download directly from S3 (avoiding bandwidth costs on backend and Vercel).

Files are stored gzipped on S3 with Content-Encoding metadata (no extra .gz suffix on the S3 key). Browsers and HTTP clients automatically decompress during download, saving 30-60% bandwidth.

Args: filename: Name of the release file to download request: FastAPI request object (to access app state)

Returns: RedirectResponse to presigned S3 URL (browser downloads directly)

Examples: GET /v1/releases/qualia-v1.2.3-macos-apple-silicon.app

Response: 307 Redirect to https://bucket.s3.amazonaws.com/qualia-v1.2.3-macos-apple-silicon.app?X-Amz-...