Fetch a stored blob

Returns the blob's raw bytes. A blob id ALONE is never sufficient: pass either the t link token returned by /api/v1/pull (valid 1 hour, and the only path that works without an account) or an account credential that owns the blob (valid for the blob's whole 24-hour life). An unknown blob, an expired blob, and one belonging to somebody else all return the same 404 — distinguishing them would confirm that an id exists. This endpoint is also rate-limited PER IP, 120 requests a minute per IP and 20000 a minute relay-wide, answering 429 above that: it is reachable with no credential and both checks above happen only after a storage read.