Manual domain checks (Recheck button / diff page load) never wrote
domains.last_check_status - only the scheduler did, leaving a
newly-templated domain stuck at "unknown" until the next scheduled run.
Extract status derivation into internal/service (single source of truth):
StatusUnknown/InSync/Drift/Error constants and DeriveStatus(diff.Changeset).
The scheduler now aliases these constants instead of duplicating them.
handleCheck persists the derived status (or StatusError on failure) via
TenantStore.SetDomainStatus after every manual check - status/history only,
no notification, which remains the scheduler's job.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3
