refactor: decompose monitor.go and sqlstore.go into focused files
Split monitor.go (1131→350 lines) into alerts.go, checks.go, sites.go, and maintenance.go by concern. Split sqlstore.go (833→470 lines) into sqlstore_alerts.go, sqlstore_history.go, and sqlstore_maintenance.go by domain. Tests move with their implementation. Pure reorganization — no behavioral changes.
This commit was merged in pull request #154.
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