docs: Add comprehensive product roadmap with milestones through Q3 2026

- Document all Phase 1 completed features (current state)
- Define Q1 2026: Infrastructure & Monitoring (issues #4, #5, #7)
- Define Q2 2026: Advanced ML Features (issues #1, #2, #6)
- Define Q3 2026: Personalization (issue #3)
- Include success metrics and technology stack
- Link to GitHub milestones and project board

Author: ringo380