feat: post engagement sync + comments fetching + error logging
- Add get_post_engagement() to fetch live reactions/comments/shares from LinkedIn
- Analytics endpoint now fetches real-time data on-demand and persists to DB
- Add sync_published_post_engagement background job (every 2h)
- Add detailed error logging to Phoenix spans (response body, error code, message)
- Write LinkedIn API errors to data/linkedin_api_errors.log for debugging