feat(admin): maintenance windows — schedule once, assign many hosts, notify
- maintenance_windows + host pivot + notification ledger; derived state (never
stored); affected-customer + banner queries live off instances
- admin /maintenance: create draft/publish, multi-host select, impact counts,
cancel; capability-gated (maintenance.manage)
- publish emails affected customers once (queued Mailable, ledger-idempotent)
- customer portal maintenance banner (upcoming <=72h + active) merged per window
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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