fix(scheduler): stop monitoring deprecated/destroyed targets

ListEnabledCheckDefs now LEFT JOINs the target entity and excludes rows whose
target is deprecated or destroyed, so retired things (secrets-issuance,
homelab-mcp, the dead secrets ingress route) stop generating permanent false
alarms instead of waiting for an operator to disable the check_def by hand.

coverageSweep's None() branch previously did nothing, so a type changed from
declared monitoring to `monitoring: none` (dns-zone) left its open
`unmonitored` signals lingering forever — a None() entity never gains a check,
so the hasCheck resolution path never fired. It now resolves those signals.

internal/db/sqlcgen/operations.sql.go

@@ -696,7 +696,9 @@ SELECT cd.entity_id, cd.target_id, cd.target_type, cd.kind, cd.config,
        e.slug AS entity_slug
 FROM check_defs cd
 JOIN entities e ON e.id = cd.entity_id
+LEFT JOIN entities tgt ON tgt.id = cd.target_id
 WHERE cd.enabled = true
+  AND (tgt.id IS NULL OR tgt.state NOT IN ('deprecated', 'destroyed'))
   AND (cd.last_run_at IS NULL
        OR cd.last_run_at <= now() - make_interval(secs => cd.interval_s))
 `