[Backend] Transaction Analytics API

[Olowodarey]

📋 Description

Build detailed transaction analytics showing spending patterns and trends.

Acceptance Criteria

• GET /analytics/transactions/summary with date range
• Breakdown by transaction type (deposit, withdrawal, interest)
• Average transaction size and frequency
• Peak transaction times/days
• Month-over-month comparison
• Export to CSV/PDF
• Chart-friendly data format

Tasks

• Create transaction analytics endpoint
• Implement breakdown calculations
• Add trend analysis
• Implement export functionality
• Format for charts
• Write tests