Add resilient reminder retry monitoring

[wognsdl2]

Summary

• Adds retry metadata to reminders (retry_count, max_attempts, last_error).
• Keeps failed reminder sends unsent, schedules bounded exponential backoff, and marks exhausted jobs when attempts are used up.
• Adds /reminders/jobs/health so operators can monitor due and exhausted reminder jobs.
• Updates schema, OpenAPI docs, README, and backend tests.

Verification

• python -m black --check packages\backend\app\routes\reminders.py packages\backend\app\models.py packages\backend\app\__init__.py packages\backend\tests\test_reminders.py
• python -m flake8 packages\backend\app\routes\reminders.py packages\backend\app\models.py packages\backend\app\__init__.py packages\backend\tests\test_reminders.py

Note: local pytest execution is blocked in this Windows environment because the existing auth test fixture attempts to reach Redis at redis:6379 without Docker/Redis available. The new retry behavior is covered by tests added in tests/test_reminders.py.

Closes #130