feat: add smart weekly financial digest with AI insights

[bbb-build]

Summary

Implements Issue #121: Smart digest with weekly financial summary — a comprehensive weekly financial digest system that aggregates expenses, income, and trends, then generates AI-powered insights.

What's included

Backend (Flask/Python):

• WeeklyDigest SQLAlchemy model with week bounds, financial totals, category breakdown, trend data, and AI insights
• services/digest.py — Full digest generation pipeline: aggregation → trend computation → AI insights (Gemini with heuristic fallback)
• 4 REST endpoints (GET /digest/weekly, GET /digest/weekly/<date>, GET /digest/history, POST /digest/generate)
• Redis caching (10-min TTL), upsert-on-regenerate, JWT-protected
• PostgreSQL schema + indexes for efficient queries

Frontend (React/TypeScript):

• Digest page with summary cards (income, expenses, net flow + trend arrows)
• Top spending categories with visual breakdown
• AI insights display section
• Week-over-week comparison grid
• Past digests browser with week navigation
• Integrated into app routing and navbar

Tests:

• 12 pytest cases covering generation, trends, caching, auth, edge cases (empty weeks, invalid dates)

Technical decisions

• Follows existing patterns exactly (dashboard.py structure, ai.py integration, cache.py usage)
• Heuristic fallback when Gemini API is unavailable
• Unique constraint on (user_id, week_start) prevents duplicate digests
• Week boundaries: Monday 00:00 → Sunday 23:59:59

Resolves #121

/claim #121