Implements admin-only broadcast posts with 48h sticky positioning in feeds:
- Migration 0004: post_type column, sticky_until timestamp, sticky feed index
- POST /api/v1/social/broadcast endpoint (admin-only)
- GET /api/v1/social/feed and channels/:id/posts now order sticky-first
- Socket.io event social:broadcast on creation
- Audit event social-broadcast-created
Part of BARAAA-95 broadcast consultation feature.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
