feat: add weekly financial digest

[JustRampage]

Summary

• Add a deterministic weekly smart digest service for FinMind.
• Expose GET /insights/weekly-summary with Monday-Sunday week normalization and optional currency filtering.
• Include income, expenses, net flow, previous-week delta, daily buckets, category shares, top expenses, upcoming bills, deterministic insights, and recommendations.
• Surface the digest in the existing Analytics page with weekly summary cards, daily flow, top expenses, upcoming bills, insights, and recommendations.
• Update OpenAPI docs and README.

Validation

• python3 -m compileall packages/backend/app packages/backend/tests
• PYTHONPATH=packages/backend uv run --python 3.12 --with-requirements packages/backend/requirements.txt black --check packages/backend/app/services/weekly_digest.py packages/backend/app/routes/insights.py packages/backend/tests/test_insights.py
• PYTHONPATH=packages/backend uv run --python 3.12 --with-requirements packages/backend/requirements.txt pytest packages/backend/tests/test_insights.py -q
• git diff --check
• direct Flask smoke test for /insights/weekly-summary?week_start=<monday>&currency=USD
• npm --prefix app run lint
• npm --prefix app run build

Targeted backend pytest result: 5 passed, 7 warnings in 0.35s.

Note: I can add a short screen-recorded demo if the reviewer wants a separate media link; the endpoint and UI behavior are covered by the validation above.

For #121
/claim #121