feat: reminder reliability tracking & delivery metrics (#123)

[alexchenai]

Summary

Implements Reminder Reliability Tracking & Delivery Metrics (Issue #123).

Changes

New Model: ReminderDeliveryLog

• Tracks every reminder delivery attempt with: status, channel, latency_ms, error_code, retry_count, attempted_at
• Linked to existing Reminder model via foreign key

New Service: ReminderReliabilityService

• get_delivery_metrics(user_id, days, channel): Overall success rate, failure rate, avg latency, p95 latency, per-channel breakdown, error code breakdown
• get_reminder_metrics(user_id, reminder_id): Per-reminder reliability score + full delivery history
• get_failed_reminders(user_id): List reminders with recent failures for retry workflows
• record_delivery_attempt(): Record a new delivery attempt with all metadata

New Routes (under /reminders prefix)

• GET /reminders/metrics?days=30&channel=email
• GET /reminders/metrics/{reminder_id}
• GET /reminders/failed?limit=50
• POST /reminders/{id}/delivery-log

Tests

15 unit tests covering:

• Empty state defaults
• All delivered / mixed success+failure
• Latency percentile calculation (p95)
• Per-channel filtering and breakdown
• Error code aggregation
• Per-reminder reliability score
• Delivery log recording (success + failure)
• Deduplication in failed reminders list

Closes #123