feat: Weekly Financial Summary Digest

[Drgonzoh]

Description

This PR implements the smart weekly financial digest feature as requested in #121.

Changes:

Backend:

• Added weekly_digest service in packages/backend/app/services/ai.py to aggregate weekly financial data (totals, category breakdown, daily spend, top expenses, and heuristic insights).
• Added GET /insights/weekly-digest endpoint in packages/backend/app/routes/insights.py with caching support (10-minute TTL).
• Added cache keys for weekly digest in packages/backend/app/services/cache.py.
• Updated packages/backend/app/openapi.yaml and README.md with the new endpoint.

Frontend:

• Added WeeklyDigest type and getWeeklyDigest API call in app/src/api/insights.ts.
• Created a new WeeklyDigest page (app/src/pages/WeeklyDigest.tsx) with summary cards, insights list, category breakdown progress bars, and top expenses table.
• Added navigation to the Weekly Digest page in Navbar.tsx.
• Registered the new route in App.tsx.

Testing:

• Added 10 new test cases in packages/backend/tests/test_insights.py covering various scenarios (empty week, populated week, WoW change, income tracking, net flow, invalid format).
• Updated packages/backend/tests/conftest.py to use fakeredis for tests to ensure they are runnable without a live Redis server.
• Added fakeredis to requirements.txt.

How to Test:

1. Run backend tests: ='.'; pytest packages/backend/tests/test_insights.py.
2. Start the frontend and navigate to the new 'Weekly Digest' tab.
3. Use the week selector to view summaries for different weeks.

/claim #121