feat: add weekly financial summary digest

[Jorel97]

/claim #121

Summary

• Added authenticated GET /insights/weekly-summary for a smart weekly digest.
• Aggregates income, expenses, net flow, transaction counts, average daily expense, daily buckets, category shares, top expenses, and upcoming bills for the selected ISO week.
• Compares weekly expenses with the previous week and returns deterministic insights/recommendations without an LLM or scheduler dependency.
• Surfaced the weekly digest on the existing Analytics page with week selection, summary cards, category share bars, daily flow, insights, and recommendations.
• Updated README and OpenAPI docs.
• Added backend tests for weekly aggregation, week normalization, and invalid dates; updated Analytics integration coverage.
• Replaced test Redis dependency with a lightweight in-memory fake so auth/cache-backed tests can run without an external Redis server.

Demo

https://github.com/Jorel97/bounty-demo-assets/raw/main/finmind/finmind-121-weekly-summary-demo.mp4

Validation

• python -m pytest packages/backend/tests/test_weekly_summary.py packages/backend/tests/test_insights.py -q -> 6 passed
• python -m black --check packages/backend/app/services/weekly_summary.py packages/backend/app/routes/insights.py packages/backend/tests/test_weekly_summary.py packages/backend/tests/conftest.py -> passed
• python -m flake8 packages/backend/app/services/weekly_summary.py packages/backend/app/routes/insights.py packages/backend/tests/test_weekly_summary.py packages/backend/tests/conftest.py -> passed
• OpenAPI parse check -> passed
• npm test -- Analytics.integration.test.tsx --runInBand -> 2 passed
• npm run build -> passed