feat: add explainable spending insights endpoint

[Mohye24k]

Summary

• Add GET /spending-insights endpoint that analyzes spending by category
• Detect spending spikes (>30% increase) and drops (>30% decrease) compared to previous period
• Warn when a single category exceeds 40% concentration of total spending
• Return insights with human-readable explanations, category breakdowns, total_spent, and daily_average
• Add comprehensive test suite (auth required, empty state, with data, daily average)
• Add TypeScript API client (app/src/api/spendingInsights.ts)
• Register blueprint in __init__.py

Test plan

• Auth required returns 401
• Empty state returns zeroed response
• With data detects spikes correctly
• Daily average calculation is correct

/claim #89
Fixes #89