feat(savings): Savings opportunity detection engine (#119)

[dagangtj]

Summary

Intelligent engine that analyzes spending patterns to identify potential savings opportunities.

Closes #119

Changes

• Detection service: packages/backend/app/services/savings_detection.py

  • Duplicate subscription detection (streaming, music, cloud, news, fitness)
  • Price increase detection on recurring charges (≥10% trigger)
  • High frequency spending alerts (>15 transactions/quarter)
  • Category spending spike detection (>50% increase vs prior period)
  • Negotiable bill identification (insurance, internet, phone, etc.)
  • Round-up-to-dollar savings calculator
  • Priority ranking: high/medium/low per opportunity

• API: packages/backend/app/routes/savings.py

  • POST /savings/opportunities — full analysis with potential savings

• Tests: 14 test cases

Example Response

{
  "opportunities": [{"type": "duplicate_subscription", "title": "Multiple streaming subscriptions", "potential_savings": 12.00}],
  "total_potential_monthly_savings": 24.50,
  "round_up_savings_monthly": 8.30,
  "opportunity_count": 3
}

Acceptance Criteria

• Production ready implementation
• Includes tests (14 cases)
• Follows existing code patterns