watcher: change messages name only the record types that differ (closes #219)

A Record Change notification printed the nameserver's whole old and new
record sets in Go map syntax, and an Inconsistency notification the two
nameservers' whole sets, so a one-address change had to be found by eye
among kilobytes of unchanged TXT, CAA, MX and NS values. Both now list,
in sorted order of type, only the record types whose values differ: a
line naming the type, then each side's values separated by commas, or
none when that side has no records of that type. The dashboard's Recent
alerts shows the same text.

Model: opus-5-5

internal/watcher/watcher.go

@@ -570,10 +570,12 @@ func (w *Watcher) detectRecordChanges(
 		}
 
 		msg := fmt.Sprintf(
-			"Hostname: %s\nNameserver: %s\n"+
-				"Old: %v\nNew: %v",
+			"Hostname: %s\nNameserver: %s\n%s",
 			hostname, ns,
-			prevNS.Records, cur.Records,
+			recordDifferences(
+				"Old", prevNS.Records,
+				"New", cur.Records,
+			),
 		)
 
 		w.notify.SendNotification(
@@ -661,10 +663,12 @@ func (w *Watcher) detectInconsistencies(
 		ns1, ns2 := pair[0], pair[1]
 
 		msg := fmt.Sprintf(
-			"Hostname: %s\n%s: %v\n%s: %v",
+			"Hostname: %s\n%s",
 			hostname,
-			ns1, current.RecordsByNameserver[ns1].Records,
-			ns2, current.RecordsByNameserver[ns2].Records,
+			recordDifferences(
+				ns1, current.RecordsByNameserver[ns1].Records,
+				ns2, current.RecordsByNameserver[ns2].Records,
+			),
 		)
 
 		w.notify.SendNotification(
@@ -1214,6 +1218,54 @@ func recordsEqual(
 	return true
 }
 
+// recordDifferences describes, in sorted order of type, each record
+// type whose values differ between a and b: a line naming the type,
+// then a line with a's values after labelA and one with b's after
+// labelB. Types with the same values in both are left out.
+func recordDifferences(
+	labelA string, a map[string][]string,
+	labelB string, b map[string][]string,
+) string {
+	types := make([]string, 0, len(a)+len(b))
+
+	for recordType := range a {
+		types = append(types, recordType)
+	}
+
+	for recordType := range b {
+		if _, ok := a[recordType]; !ok {
+			types = append(types, recordType)
+		}
+	}
+
+	sort.Strings(types)
+
+	var lines []string
+
+	for _, recordType := range types {
+		if sliceEqual(a[recordType], b[recordType]) {
+			continue
+		}
+
+		lines = append(lines,
+			"Type: "+recordType,
+			labelA+": "+joinValues(a[recordType]),
+			labelB+": "+joinValues(b[recordType]),
+		)
+	}
+
+	return strings.Join(lines, "\n")
+}
+
+// joinValues lists record values separated by commas, or says none.
+func joinValues(values []string) string {
+	if len(values) == 0 {
+		return "none"
+	}
+
+	return strings.Join(values, ", ")
+}
+
 func sliceEqual(a, b []string) bool {
 	if len(a) != len(b) {
 		return false