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

An http target's destination is frequently a Slack, Discord or
Teams incoming-webhook endpoint whose path segments are the
credential — the same property that made the Slack target's
webhook URL a bearer token. The source detail page rendered it
in full, so the leak closed for slack targets stayed reachable
through a different target type.

Render it through the existing MaskURL, which reduces a URL to
scheme and host. The field accepts an arbitrary URL, so no path
segment can be assumed non-secret and none is shown.

internal/handlers/source_detail_test.go

@@ -131,6 +131,47 @@ func TestHandleSourceDetail_MasksSlackWebhookURL(t *testing.T) {
 	assert.Contains(t, body, "https://hooks.slack.com/...")
 }
 
+// TestHandleSourceDetail_MasksHTTPDestinationURL is the
+// regression test for the same leak reached through the http
+// target: its destination is routinely an incoming-webhook
+// endpoint whose path segments are the credential, so the
+// rendered page must not contain them.
+func TestHandleSourceDetail_MasksHTTPDestinationURL(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	var (
+		h    *handlers.Handlers
+		sess *session.Session
+		db   *database.Database
+	)
+
+	app := newTestApp(t, &h, &sess, &db)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	wh := seedWebhook(t, db)
+	seedConfiguredTarget(
+		t, db, wh.ID,
+		database.TargetTypeHTTP,
+		`{"url":"`+slackWebhookURL+`"}`,
+	)
+
+	body := renderSourceDetailPage(t, h, sess, wh.ID)
+
+	assert.NotContains(t, body, slackSecretPath)
+	assert.NotContains(t, body, "T00000000")
+	assert.NotContains(t, body, "B00000000")
+	assert.NotContains(
+		t, body, "XXXXXXXXXXXXXXXXXXXXXXXX",
+	)
+
+	assert.Contains(t, body, "Destination URL")
+	assert.Contains(t, body, "https://hooks.slack.com/...")
+}
+
 // TestHandleSourceDetail_RendersNamedTargetFields proves the
 // other target types render labelled fields rather than the
 // stored blob.
@@ -172,7 +213,7 @@ func TestHandleSourceDetail_RendersNamedTargetFields(
 	body := renderSourceDetailPage(t, h, sess, wh.ID)
 
 	assert.Contains(t, body, "Destination URL")
-	assert.Contains(t, body, "https://example.com/hook")
+	assert.Contains(t, body, "https://example.com/...")
 	assert.Contains(t, body, "Timeout")
 	assert.Contains(t, body, "1 configured")
 	assert.NotContains(t, body, "sekrit")