Generate AI post variants

Generates up to count post-copy variants per requested platform via the assisters.dev AI gateway, and persists them as draft post variants (so they also count toward the monthly generation quota and are listable via GET /drafts). Rate limit: 20 requests/minute. Requires an active or trialing plan: without one the call returns 402 with code: "plan_limit_exceeded", reason: "no_active_plan" and the plan offer (MisarPost has no Free plan). A plan's monthly generation cap, where it has one, also returns 402 with code: "plan_limit_exceeded" and an upgrade offer.