Get attribution certificate

Get a signed attribution certificate for a completed job.

Returns a deterministic, HMAC-signed certificate containing the full analysis results, chain-of-evidence, and cryptographic metadata for independent verification.

Implements generate-once/serve-forever: the first request creates and persists the certificate, subsequent requests return the stored version.

Returns 404 if the job does not exist, 409 if the analysis is still in progress, and the signed certificate envelope when complete.