Add reminder delivery reliability metrics

[alceops]

Summary

• adds reminder delivery reliability fields (status, attempts, timestamps, last error) to the model/schema with PostgreSQL compatibility migration
• records delivered vs failed outcomes from /reminders/run instead of marking all attempts as sent
• adds authenticated /reminders/reliability aggregate metrics by total and channel
• includes Redis outage-safe fallback behavior so auth/cache paths continue to work in local/free-tier environments without Redis
• documents the reliability endpoint and fields

Tests

• PYTHONPATH=. ../../.venv/bin/pytest tests (23 passed)

Closes #123