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/scheduler/coverage.go

@@ -85,7 +85,16 @@ func coverageSweep(ctx context.Context, pool *db.Pool) {
 		case !mon.Declared:
 			undeclared++
 		case mon.None():
-			// Explicitly unmonitorable. Nothing to say.
+			// Explicitly unmonitorable. Nothing to raise — but a type that
+			// USED to declare monitoring (e.g. dns-zone, [dns]→none) may have
+			// open `unmonitored` signals from before the change. They are no
+			// longer a gap, so close them; otherwise they linger forever,
+			// because resolveCoverageSignal only runs from the hasCheck path
+			// and a None() entity never gains a check.
+			if resolveCoverageSignal(ctx, pool, e.id) {
+				resolved++
+				slog.Info("scheduler: type now unmonitorable, resolving stale signal", "entity", e.slug)
+			}
 		case e.hasCheck:
 			if resolveCoverageSignal(ctx, pool, e.id) {
 				resolved++