Replace a workflow's tags

REPLACES the whole tag set — send the complete list you want, not a delta; {"tags": []} clears every tag. To merge instead, use POST /api/workflows/bulk with op: addTags.

This is a dedicated column write: it never touches the definition, never mints a version and never bumps definitionRev, so it is safe on a live workflow and cannot conflict with an in-flight editor save. Tags are trimmed, blank entries dropped, and deduplicated case-insensitively with the first spelling kept — so ["Prod", "prod"] stores one tag, Prod. The response is a bare acknowledgement, not the workflow: read the tags back with GET /api/workflows/{id} if you need them.