Cancel Enrichment Workflows Bulk

Bulk-cancel in-flight DBOS enrichment workflows.

Best-effort + idempotent, matching the single-cancel contract. Returns 204 unconditionally — workflows the caller doesn't own, workflows without a reservation row, and workflows already in a terminal state are silently skipped. The FE's column-level cancel sends every workflow_id it tracked and relies on polling to converge each one to REVOKED.