Add weekly financial digest endpoint

[kishan5111]

Summary

• add authenticated weekly digest endpoint with income, spending, net flow, trend comparison, top categories, largest transactions, and upcoming bills
• document the endpoint in OpenAPI and README
• add focused backend tests for successful digest generation and invalid end_date handling

Tests

• ../../.venv/bin/python -m pytest tests/test_weekly_digest.py -q
• .venv/bin/python -m black --check packages/backend/app/services/weekly_digest.py packages/backend/app/routes/insights.py packages/backend/tests/test_weekly_digest.py
• .venv/bin/python -m flake8 packages/backend/app/services/weekly_digest.py packages/backend/app/routes/insights.py packages/backend/tests/test_weekly_digest.py
• git diff --check

/claim #121