feat: add email notification digests for anomalous spending

[Sagargupta16]

Summary

Wire up email notifications for the existing anomaly detection system, so users get proactive alerts without opening the app.

What Already Exists

• Anomaly table with types: high_expense, unusual_category, large_transfer, budget_exceeded
• _detect_anomalies() in analytics engine with threshold-based detection
• /api/analytics/v2/anomalies endpoint + review/dismiss workflow
• AnomalyReviewPage in frontend with severity levels
• notify_anomalies field in user preferences (not wired up)

What's Missing

1. Email Delivery

• Integrate a transactional email service (Resend or SendGrid free tier)
• Daily digest: batch all new anomalies into one email
• Clean email template showing flagged transactions with context

2. Notification Preferences

• Wire up existing notify_anomalies preference to actually trigger emails
• Add email input field in Settings
• Threshold configuration for sensitivity

3. Rate Limiting

• Max 1 digest email per day
• Never include sensitive account details in emails

Acceptance Criteria

• Opted-in users receive daily digest emails for new anomalies
• Email template is clean and informative
• Users can configure notification preferences in Settings
• No emails sent without explicit opt-in