Add weekly financial digest

[itsjustet-lab]

Summary

• Add a JWT-protected GET /insights/weekly-summary endpoint that normalizes the requested week to Monday and returns weekly cash flow, week-over-week spend comparison, category and daily breakdowns, largest expenses, upcoming bills, highlights, insights, and recommendations.
• Wire the existing Analytics page to load the weekly digest alongside the monthly budget suggestion, with week start and currency controls.
• Add typed frontend API support plus backend and frontend coverage for the new digest flow.

Validation

• ..\..\.venv\Scripts\python -m pytest tests\test_insights.py -q
• .\.venv\Scripts\python -m black --check packages\backend\app\routes\insights.py packages\backend\tests\test_insights.py packages\backend\tests\conftest.py
• .\.venv\Scripts\python -m flake8 packages\backend\app\routes\insights.py packages\backend\tests\test_insights.py packages\backend\tests\conftest.py
• npm --prefix app test -- Analytics.integration.test.tsx
• npm --prefix app run build
• git diff --check

/claim #121

[itsjustet-lab]

weekly-financial-digest-demo.webm