Assign a tag to an applicant

Assigns a tag (by name) to this applicant. If no tag with this exact name exists yet for the company, one is created. See `GET /api/v1/lookup/applicant-tags` to discover existing tag names and avoid near-duplicates. Assigning the same tag more than once has no additional effect. The tag name is passed in the request body, not the URL path, since tag names may contain characters (spaces, slashes) that don't survive path encoding cleanly.