feat: add weekly financial digest

[peterxing]

Summary

• What changed: Added an authenticated GET /insights/weekly-digest endpoint that returns weekly income, spending, net flow, transaction count, top categories, previous-week comparison, and deterministic insight text.
• Why: Implements the weekly smart digest requested in Smart digest with weekly financial summary #121 with a production-ready backend path, docs, and tests.

Closes #121

/claim #121

Validation

• Frontend lint: cd app && npm run lint (not run; backend-only change)
• Frontend tests: cd app && npm test -- --runInBand (not run; backend-only change)
• Backend tests: cd packages/backend && .\.venv\Scripts\python -m pytest — 24 passed
• Backend lint: cd packages/backend && .\.venv\Scripts\python -m flake8 app tests
• Updated docs if needed — OpenAPI docs updated

Security and Ownership

• PR opened from a fork (not direct push to main)
• CODEOWNERS review requested

Checklist

• No secrets added
• No unrelated files changed
• Breaking changes documented — no breaking changes