Add weekly smart financial digest

[michellehumes]

Summary

Implements the weekly smart financial digest requested in #121.

/claim #121

What changed

• Added authenticated GET /insights/weekly-digest.
• Normalizes any requested date to a Monday-Sunday week.
• Supports optional currency filtering.
• Returns weekly income, expenses, net flow, transaction count, daily breakdown, category breakdown, top expenses, and previous-week comparison.
• Adds upcoming bills due during the selected week.
• Generates deterministic insights and recommendations without requiring an LLM key or scheduler.
• Surfaces the digest in the existing Analytics page.
• Updates OpenAPI docs and README.
• Adds backend coverage for the digest response and invalid week handling.

Verification

• black --check packages/backend/app/services/ai.py packages/backend/app/routes/insights.py packages/backend/tests/test_insights.py
• npm run build
• Backend service smoke check with in-memory SQLite covering:
  • weekly expenses
  • previous-week comparison
  • upcoming bill inclusion
  • deterministic insights

Local test note

Focused pytest is blocked in my local environment before feature execution because the auth fixture/login path attempts to connect to Redis hostname redis, and there is no local Redis server/binary available. The new digest service was smoke-tested directly with the same models and in-memory SQLite.

Fixes #121