[Backend] Savings Performance Report API

[Olowodarey]

📋 Description

Generate detailed performance reports for user's savings portfolio with benchmarking.

Acceptance Criteria

• GET /analytics/savings/performance with date range
• Calculate actual vs. projected returns
• Compare performance across different products
• Benchmark against platform averages
• Risk-adjusted returns calculation
• Identify best and worst performing products
• Downloadable PDF report

Tasks

• Create performance report endpoint
• Implement return calculations
• Add benchmarking
• Implement PDF generation
• Add product comparison
• Write tests