Add weekly financial digest

[partyplatter08-lab]

Summary

• Add authenticated GET /insights/weekly-summary for an on-demand Monday-Sunday financial digest.
• Include income, expenses, net flow, savings rate, previous-week comparison, daily totals, category breakdowns/trends, largest expenses, due bills, highlights, insights, and recommendations.
• Surface the digest in the existing Analytics page with week-start and currency controls.
• Format the default Week Start from local date fields so the page does not request a non-Monday in negative UTC offsets.
• Update OpenAPI and README docs, and add backend/frontend tests.

Assumptions

• The weekly digest is deterministic and generated from existing transactions and bills; it does not add a scheduler or LLM dependency.
• If currency is omitted, the backend uses the user's preferred currency.

Validation

• cd packages/backend && python3 -m pytest -q -> 26 passed
• cd packages/backend && python3 -m black --check app tests -> passed
• cd packages/backend && python3 -m flake8 app tests -> passed
• cd packages/backend && python3 -c "import yaml; yaml.safe_load(open('app/openapi.yaml')); print('openapi yaml ok')" -> passed
• cd app && npm test -- --runInBand -> 29 passed
• cd app && npm run lint -> passed
• cd app && npm run build -> passed
• Playwright browser smoke with backend-like Monday validation -> initial week_start=2026-05-11, refresh to week_start=2026-05-04&currency=USD, rendered 2026-05-04 to 2026-05-10 (USD), no page errors

Demo

• Attach analytics-weekly-digest-demo.mp4, a short demo video showing the Analytics weekly digest load, currency refresh, and week-start refresh flow.

/claim #121
Closes #121