feat: add weekly financial digest

[yanziwei]

/claim #121

Summary

• adds an authenticated GET /insights/weekly-summary endpoint that normalizes any requested date to the ISO week Monday
• returns weekly income, expenses, net flow, daily buckets, category shares, week-over-week deltas, top expenses, upcoming bills, and deterministic insight messages
• documents the endpoint in the README and OpenAPI spec, and exposes a typed frontend API client helper
• makes backend tests independent from external Redis by replacing the test Redis client with an in-memory stub

Validation

• ../../.venv/bin/python -m black --check app tests
• ../../.venv/bin/python -m pytest -q tests/test_expenses.py tests/test_bills.py tests/test_insights.py
• npm run lint
• npm test -- --runInBand app/src/__tests__/apiClient.test.ts

[yanziwei]

Demo

Short demo video: https://github.com/yanziwei/FinMind/releases/download/finmind-pr-1185-demo/finmind-weekly-summary-demo.mp4

Demo transcript: https://github.com/yanziwei/FinMind/releases/download/finmind-pr-1185-demo/finmind-weekly-summary-demo.txt

What it shows:

• registers and logs in a demo user
• creates categories, current-week expenses, previous-week expenses, and an upcoming bill through authenticated API calls
• calls GET /insights/weekly-summary?week_start=2026-03-05
• verifies the response normalizes the week to 2026-03-02 through 2026-03-08 and returns totals, category trends, previous-week deltas, top expenses, upcoming bills, and insight messages

The transcript redacts JWT values; all displayed response data is generated from the demo API run.