Start a taxonomy run

Starts a manual taxonomy generation run for a field or directory scope. Hub validates that the scope has enough embedded text feedback (below the configured minimum returns 400 with an "insufficient data" validation error), creates the run, and hands it to the taxonomy compute service. Omit scope_type for the existing field scope behavior; use scope_type=directory with tenant_id only to generate one taxonomy over all text feedback in the directory.

Idempotent per scope: if a run is already pending or running for the same scope, the existing run is returned with in_progress: true (HTTP 200) instead of starting a new one; a newly created run returns HTTP 202 with in_progress: false. While a tenant data purge runs for the same tenant_id, the request is rejected with HTTP 409 (code tenant_write_conflict) and may be retried. Requires Hub embeddings and the taxonomy service to be configured; otherwise returns 503.