Mask the http target's destination URL in the UI (closes #115)

The http target's destination URL can itself be a bearer credential, and
the source detail page rendered it in full. Render it through the
existing MaskURL instead, matching the rule already applied to slack
targets.

Independently reviewed: mutation-verified (reverting to the raw value
fails the absence assertions, not merely the masked-form ones), MaskURL
probed against userinfo, query, fragment, port, IPv6 literal and
non-http schemes, and every sibling path that surfaces target data
re-walked and found clean.

internal/delivery/target_config_view_test.go

@@ -19,6 +19,7 @@ const (
 
 	viewExampleOrigin = "https://example.com"
 	viewExampleHook   = viewExampleOrigin + "/hook"
+	viewMaskedOrigin  = viewExampleOrigin + "/..."
 	viewUnavailable   = "(unavailable)"
 	viewExpiryNever   = "never"
 )
@@ -162,7 +163,7 @@ func TestNewTargetViews_HTTP(t *testing.T) {
 	assert.Equal(
 		t,
 		map[string]string{
-			"Destination URL": viewExampleHook,
+			"Destination URL": viewMaskedOrigin,
 			"Timeout":         "30s",
 			"Headers":         "1 configured",
 			"Max Retries":     "5",
@@ -188,13 +189,41 @@ func TestNewTargetViews_HTTPFireAndForget(t *testing.T) {
 	assert.Equal(
 		t,
 		map[string]string{
-			"Destination URL": viewExampleHook,
+			"Destination URL": viewMaskedOrigin,
 			"Max Retries":     "0 (fire-and-forget)",
 		},
 		fieldMap(view.Config),
 	)
 }
 
+// TestNewTargetViews_HTTPMasksDestinationURL proves the rule
+// holds for the http target too: an http destination is
+// routinely an incoming-webhook endpoint whose path segments
+// are the credential, so none of them is shown.
+func TestNewTargetViews_HTTPMasksDestinationURL(t *testing.T) {
+	t.Parallel()
+
+	view := viewFor(t, database.Target{
+		Type:   database.TargetTypeHTTP,
+		Config: `{"url":"` + slackWebhookURL + `"}`,
+	})
+
+	fields := fieldMap(view.Config)
+
+	assert.Equal(
+		t,
+		"https://hooks.slack.com/...",
+		fields["Destination URL"],
+	)
+
+	for _, v := range fields {
+		assert.NotContains(t, v, slackSecretPath)
+		assert.NotContains(t, v, "T00000000")
+		assert.NotContains(t, v, "B00000000")
+		assert.NotContains(t, v, "XXXXXXXXXXXXXXXXXXXXXXXX")
+	}
+}
+
 func TestNewTargetViews_Database(t *testing.T) {
 	t.Parallel()