feat: add weekly smart digest financial summary endpoint

[lustsazeus-lab]

Summary

• add GET /insights/weekly-summary for a 7-day financial digest
• include totals, week-over-week trends, top categories, daily net flow, and actionable insights
• expose weekly summary types/helpers in frontend API client
• document endpoint in README and OpenAPI spec

Validation

• python3 -m py_compile packages/backend/app/services/ai.py packages/backend/app/routes/insights.py packages/backend/tests/test_insights.py
• exercised endpoint with in-memory SQLite + Flask test client (/insights/weekly-summary?end_date=2026-03-01 returned 200 and expected totals/trends)
• note: full pytest run currently needs Redis host wiring in local env

Closes #121