[Enhancement] Add automatic cleanup for expired authentication nonces

[giwaov]

Description

Expired authentication nonces accumulate in the database over time, potentially causing storage bloat and performance degradation.

Current Behavior

• Nonces are created for each SIWE authentication attempt
• Nonces expire after 5 minutes
• Used and expired nonces remain in the database indefinitely

Expected Behavior

• Expired and used nonces should be automatically cleaned up
• Database should not accumulate stale authentication data

Impact

• Database storage grows unnecessarily
• Potential performance impact on nonce lookups
• Stale data in production database

Suggested Solution

A management command has been proposed in PR #450 that provides:

# Clean up nonces expired more than 1 hour ago
python manage.py cleanup_nonces

# Preview what would be deleted
python manage.py cleanup_nonces --dry-run

# Custom age threshold
python manage.py cleanup_nonces --hours 24

This should be scheduled to run periodically (daily recommended) via:

• Cron job
• Django-celery-beat
• AWS CloudWatch Events (for App Runner deployments)

Code Location

• Model: backend/ethereum_auth/models.py - Nonce class
• Views: backend/ethereum_auth/views.py - nonce creation/validation