Return the canonical health verdict for ?uri=.

Query parameters:

• uri (required) - the artifact key.
• at - evaluation instant; defaults to the wall clock. Pin it to reproduce a verdict byte-for-byte against the CLI.
• cadence_seconds - declared refresh cadence; omitted means the cadence leg reports not_applicable.

The body is the exact string the CLI prints for the same state and instant, byte for byte. The status is always 200: the verdict is the payload, and a red artifact is a successfully computed answer, not a failed request.