Delete (and rename) tags team-wide #150

Tags only exist on contacts. You can remove a tag from one contact (DELETE /contacts/:contactId/tags/:tag), but you can't delete or rename a tag across the whole team. A mistyped or retired tag stays forever and keeps appearing in filters and segment builders.

Scope

• API: list the team's tags with contact counts, delete a tag from all contacts, and rename a tag (optional). Show which segments, sequence triggers and broadcast filters use the tag before deleting, and block or warn. Update the OpenAPI docs.
• MCP: matching tools.
• Dashboard: a tags management view (shadcn table) with Delete and a confirmation that shows the number of affected contacts and references.