Fix Previously up services are alerted as going back up if the master goes down #1

This gets rid of the alert on unknown -> up, will still alert unknown -> down by design.

internal/alerts/dispatcher_test.go

@@ -0,0 +1,30 @@
+package alerts
+
+import (
+	"testing"
+
+	"git.cer.sh/axodouble/quptime/internal/checks"
+)
+
+func TestShouldAlertFiltersColdStartUp(t *testing.T) {
+	cases := []struct {
+		name string
+		from checks.State
+		to   checks.State
+		want bool
+	}{
+		{"cold start to up (master failover / daemon restart)", checks.StateUnknown, checks.StateUp, false},
+		{"cold start to down still alerts", checks.StateUnknown, checks.StateDown, true},
+		{"real recovery alerts", checks.StateDown, checks.StateUp, true},
+		{"regression alerts", checks.StateUp, checks.StateDown, true},
+		{"stale (up to unknown) suppressed", checks.StateUp, checks.StateUnknown, false},
+		{"stale (down to unknown) suppressed", checks.StateDown, checks.StateUnknown, false},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := shouldAlert(c.from, c.to); got != c.want {
+				t.Errorf("shouldAlert(%s→%s) = %v, want %v", c.from, c.to, got, c.want)
+			}
+		})
+	}
+}