Add weekly smart digest summary

[alceops]

Closes #121

Summary

• Add authenticated GET /insights/weekly-summary for weekly financial digests with income, expenses, net flow, week-over-week trends, top categories, upcoming bills, highlights, insights, and recommendations.
• Add Redis-unavailable in-memory fallbacks for refresh sessions/cache so local and free-tier environments degrade gracefully.
• Invalidate dashboard summary cache when expenses are created.
• Document the new endpoint in README.

Tests

• PYTHONPATH=. ../../.venv/bin/pytest tests (24 passed)