feat: add weekly financial digest insights

[wognsdl2]

Summary

• Adds a new authenticated /insights/weekly-digest endpoint for weekly income, expense, net-flow, previous-week comparison, daily totals, top categories, insights, and recommended actions.
• Exposes a frontend getWeeklyDigest API client and TypeScript response type.
• Documents the endpoint in OpenAPI and README.
• Adds focused tests for weekly digest success and invalid week_start, plus a fake Redis test client so auth/cache tests run without a live Redis container.

Verification

• python -m black --check packages/backend/app/services/ai.py packages/backend/app/routes/insights.py packages/backend/tests/test_insights.py packages/backend/tests/conftest.py
• python -m flake8 packages/backend/app/services/ai.py packages/backend/app/routes/insights.py packages/backend/tests/test_insights.py packages/backend/tests/conftest.py
• python -m pytest tests/test_insights.py -q from packages/backend

Closes #121