Reduce maintenance notification noise

[120318]

Summary

• Suppress scheduler completed events for media that already has a successful media server sync
• Keep first-time scheduler scrape events, while making later NFO/image maintenance silent
• Add a 24-hour per-site cooldown for indexer unhealthy notifications after the existing 3-failure threshold
• Persist indexer health notification timestamps with a migration

Tests

• ./aethera.sh test-backend tests/test_media_server_sync_boundaries.py tests/test_indexer_site_health_alerts.py
• ./scripts/review_with_gates.sh

[chatgpt-codex-connector]

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