dashboard, status API and notifications tell apex domains from hostnames (closes #224)

An apex domain's own records are still saved with the hostnames'
records, under the domain's name, so the port and TLS checks find its
addresses. Notifications about them now start `Domain:`, decided by the
configured domains. The dashboard and /api/v1/status, which read only
the saved state, take a hostname entry whose name also has a domain
entry as that domain's own records: the dashboard shows them in a second
table under Domains, the API in the domain's `recordsByNameserver`, and
neither lists or counts them as hostnames. The startup notification
counts domains and hostnames from the configuration. README says which
of a domain's own records are watched and how their changes are
notified.

Model: opus-5-5

internal/watcher/watcher.go

@@ -271,8 +271,9 @@ func (w *Watcher) checkDomain(
 		LastChecked:         now,
 	})
 
-	// The apex domain's records are also checked as a hostname's, so
-	// that the port and TLS checks find its addresses.
+	// The apex domain's records are also checked and saved as a
+	// hostname's, so that the port and TLS checks find its addresses.
+	// Notifications about them name it as a domain (see nameLine).
 	w.checkHostname(ctx, domain)
 }
 
@@ -541,6 +542,17 @@ func (w *Watcher) detectHostnameChanges(
 	w.detectCNAMEAddressChanges(ctx, hostname, prev, current)
 }
 
+// nameLine is the line a notification about name's records starts with:
+// "Domain: " and the name for a configured apex domain, whose own
+// records are checked as a hostname's are, and "Hostname: " otherwise.
+func (w *Watcher) nameLine(name string) string {
+	if slices.Contains(w.config.Domains, name) {
+		return "Domain: " + name
+	}
+
+	return "Hostname: " + name
+}
+
 // detectCNAMEAddressChanges notifies when the addresses at the end of
 // hostname's CNAME chain differ from those the previous check saved,
 // including a change from or to none. When the previous addresses are
@@ -557,8 +569,8 @@ func (w *Watcher) detectCNAMEAddressChanges(
 	}
 
 	msg := fmt.Sprintf(
-		"Hostname: %s\nOld: %s\nNew: %s",
-		hostname,
+		"%s\nOld: %s\nNew: %s",
+		w.nameLine(hostname),
 		strings.Join(old, ", "),
 		strings.Join(cur, ", "),
 	)
@@ -590,8 +602,8 @@ func (w *Watcher) detectRecordChanges(
 		}
 
 		msg := fmt.Sprintf(
-			"Hostname: %s\nNameserver: %s\n%s",
-			hostname, ns,
+			"%s\nNameserver: %s\n%s",
+			w.nameLine(hostname), ns,
 			recordDifferences(
 				"Old", prevNS.Records,
 				"New", cur.Records,
@@ -618,8 +630,8 @@ func (w *Watcher) detectNSDisappearances(
 		}
 
 		msg := fmt.Sprintf(
-			"Hostname: %s\nNameserver: %s disappeared",
-			hostname, ns,
+			"%s\nNameserver: %s disappeared",
+			w.nameLine(hostname), ns,
 		)
 
 		w.notify.SendNotification(
@@ -648,8 +660,8 @@ func (w *Watcher) detectNSFailures(
 		switch {
 		case prevNS.Status == statusOK && cur.Status == statusError:
 			msg := fmt.Sprintf(
-				"Hostname: %s\nNameserver: %s\nError: %s",
-				hostname, ns, cur.Error,
+				"%s\nNameserver: %s\nError: %s",
+				w.nameLine(hostname), ns, cur.Error,
 			)
 
 			w.notify.SendNotification(
@@ -660,8 +672,8 @@ func (w *Watcher) detectNSFailures(
 			)
 		case prevNS.Status == statusError && cur.Status == statusOK:
 			msg := fmt.Sprintf(
-				"Hostname: %s\nNameserver: %s recovered",
-				hostname, ns,
+				"%s\nNameserver: %s recovered",
+				w.nameLine(hostname), ns,
 			)
 
 			w.notify.SendNotification(
@@ -683,8 +695,8 @@ func (w *Watcher) detectInconsistencies(
 		ns1, ns2 := pair[0], pair[1]
 
 		msg := fmt.Sprintf(
-			"Hostname: %s\n%s",
-			hostname,
+			"%s\n%s",
+			w.nameLine(hostname),
 			recordDifferences(
 				ns1, current.RecordsByNameserver[ns1].Records,
 				ns2, current.RecordsByNameserver[ns2].Records,
@@ -1180,7 +1192,9 @@ func (w *Watcher) saveState() {
 // after the first full scan completes, if SEND_TEST_NOTIFICATION
 // is enabled. The message is informational, not an error or anomaly
 // alert. It is written before it reaches any endpoint, so it claims
-// nothing about whether the endpoints work.
+// nothing about whether the endpoints work. Domains and hostnames are
+// counted from the configuration: the state's hostnames also hold each
+// apex domain's own records.
 func (w *Watcher) maybeSendTestNotification(ctx context.Context) {
 	if !w.config.SendTestNotification {
 		return
@@ -1194,8 +1208,8 @@ func (w *Watcher) maybeSendTestNotification(ctx context.Context) {
 			"Tracking %d port endpoint(s) and %d TLS certificate(s).\n"+
 			"This is a test notification, sent to every configured "+
 			"notification endpoint.",
-		len(snap.Domains),
-		len(snap.Hostnames),
+		len(w.config.Domains),
+		len(w.config.Hostnames),
 		len(snap.Ports),
 		len(snap.Certificates),
 	)