Download File

Signed GET URL for one active file's bytes — OpenAI's "Retrieve file content" / Anthropic's "Download a File". This is the endpoint clients use to render the file: the returned get_url goes straight into an <img src> (or equivalent); bytes flow from GCS to the client directly, never through Anvil.

Member+kind-gated: wrong member, wrong kind, unknown file_id, a file still pending (no active object exists yet to sign), and an expired/reclaimed bucket object are all indistinguishable 404s — no existence oracle. Every successful call is a "use" — re-bumps custom_time/last_used_at, unlike GET /files/{file_id}'s metadata read.