Retarget Segments

Apply one targeting to a set of campaigns in a single request (T-286).

The "use the same targeting for every campaign" action: instead of the browser firing one POST /segments/{id}/retarget per campaign, this writes the same geo / company-size prompts to every selected campaign in ONE transaction, then nudges each live campaign to re-search. Mirrors the project-scoped save-templates / save-links "apply to all" pattern. Returns the per-campaign action so the client can show the same breakdown the N-call path showed.