docs: add implementation plan for SSC-14 Exam Prediction Engine

Add comprehensive implementation plan for the Exam Content Prediction
Engine feature - a key differentiator for StudySync. This plan includes:

- Database schema for ExamPrediction, TopicPrediction, StudyPlan, ActualExamResult
- Backend prediction service with AI emphasis detection
- Topic probability calculation algorithms
- Study plan generation with prioritization
- Exam readiness assessment
- Post-exam feedback loop for accuracy improvement
- Frontend prediction wizard and dashboard components
- Code examples for key services

This feature is gated to STUDENT_PLUS and UNIVERSITY tiers.

Related: SSC-14, GitHub Issue #2

Author: Sakeeb91