Permanently remove a segment

Removes the segment from every read surface: GET /v1/segments/{id} returns 404 afterward, and GET /v1/segments always excludes it (there is no override, unlike archived segments). There is no restore endpoint — this is a one-way operation from the API's perspective, even though the underlying record is retained internally rather than hard-deleted.

Effect on audiences: unlike archiving, deleting a segment that's referenced by one or more audiences changes those audiences' composition — the segment is dropped from their resolved segment list and record count. This is the reason for the impact check below; archiving has no such check because it never has this effect.

Impact check: if the segment is referenced by one or more active audiences, the server returns a 409 with audience_count, campaign_count, and an audiences list. Re-submit with ?confirm_impact=true to proceed anyway.

Not idempotent: calling this twice returns 404 on the second call, since the segment is already excluded from lookup after the first delete. No credits are debited. Requires 'creator' scope.