feat(rsocials): AI campaign generator from event brief
Add "Generate from Brief" feature: paste unstructured event text, AI
(Gemini 2.5 Pro) creates a full multi-phase, multi-platform campaign
with threads, emojis, newsletter content, and platform-specific formatting.
- Schema v4: add threadPosts, emailSubject, emailHtml to CampaignPost
- New POST /api/campaign/generate endpoint with platform-aware prompting
- Generate modal with platform checkboxes, tone/style selectors
- Preview mode with save/regenerate/discard flow
- Dynamic phase rendering (supports 3-5 AI-generated phases)
- Thread badge with expandable individual posts
- Newsletter platform support (icon + color)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>