Add A/B testing for follow-up email templates

[humayun-sarfraz]

Description

Allow the system to generate multiple email variants and track which performs best.

Features

• Generate 2-3 email variants per lead
• Track open rates and reply rates per variant
• Auto-select winning template over time
• Dashboard showing A/B test results

Technical Approach

• Store multiple FollowUpEmail records per lead with variant labels
• Randomly assign variant when sending
• Aggregate stats in analytics dashboard