feat: Smart digest with weekly financial summary (#121)

[wocaoac-cpu]

Summary

Closes #121

Implements a Smart Digest feature that generates weekly financial summaries with AI-powered insights.

New Endpoint

GET /digest/weekly?week=YYYY-Www

• JWT-protected
• Accepts optional ISO week parameter (defaults to current week)
• Supports user-supplied Gemini API key via X-Gemini-Api-Key header
• Supports custom AI persona via X-Insight-Persona header
• Results cached in Redis (10 min TTL) for non-custom requests

Response Payload

Section	Description
period	ISO week metadata with start/end dates
summary	Total income, expenses, net flow, week-over-week change %, upcoming bills total/count
category_breakdown	Per-category expense totals with percentage shares
daily_spending	Day-by-day expense trend for the week
top_transactions	Top 5 largest expenses
upcoming_bills	Bills due within the week
insights	AI highlights, risk flags, and saving tips

AI Integration

• Gemini (primary): Calls Gemini API with financial data to generate contextual insights
• Heuristic (fallback): Rule-based insights when Gemini is unavailable or no API key is configured
• Graceful degradation: Gemini failures automatically fall back to heuristic with a warnings field

Files Changed

• packages/backend/app/services/smart_digest.py — Core aggregation logic + AI enrichment
• packages/backend/app/routes/digest.py — Blueprint with /weekly endpoint
• packages/backend/app/routes/__init__.py — Register digest blueprint
• packages/backend/app/openapi.yaml — API documentation with WeeklyDigest schema
• packages/backend/tests/test_smart_digest.py — 29 tests covering:
  • parse_week_param validation (7 tests)
  • _week_bounds ISO week calculation (2 tests)
  • Integration tests for endpoint behavior (14 tests)
  • Heuristic insights logic (6 tests)

Test Plan

• Unit tests for week parsing and bounds calculation
• Unit tests for heuristic insight generation (all edge cases)
• Integration tests for endpoint structure, data accuracy, caching, auth
• Gemini integration test (mocked) verifying key passthrough and fallback
• Verified existing tests unaffected (same Redis dependency for integration tests)
• Full integration test with Docker Compose (Redis + PostgreSQL)