Delete a context_attachment

Deletes the attachment item and, if it has an associated S3 blob, the blob too. Idempotent at the HTTP layer: a delete on a non-existent attachment returns 404 with a stable error code so the integrator can choose to treat it as success.

Snapshot semantics: if extraction is in flight against this attachment, the in-memory snapshot in the worker is unaffected — the in-flight job completes against the pinned payload, and the resulting facts carry the now-dead attachment_id in Fact.context_attachment_ids_used.