Add a read-only Settings page for the loaded configuration (closes #402)

A new page at /settings, linked from the navigation bar and behind the login, lists every configuration field the server loaded at startup: its environment variable, the README table's description, and the value in effect. METRICS_PASSWORD and SENTRY_DSN show only as set or not set; their values are replaced before rendering and never reach the template. The route is GET only and its group is built like the other admin page groups. Tests set the credentials one at a time so each value shown is checked against its own field and a set secret never appears in the page.

Model: opus-5-5

internal/server/settings_test.go

@@ -0,0 +1,24 @@
+package server_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSettingsPageIsBehindLogin(t *testing.T) {
+	t.Parallel()
+
+	env := newTestEnv(t)
+
+	w := env.get("/settings", nil)
+	assert.Equal(t, http.StatusSeeOther, w.Code)
+	assert.Equal(
+		t, "/pages/login?next=%2Fsettings", w.Header().Get("Location"),
+	)
+
+	w = env.get("/settings", env.authCookies(t, "id", "admin"))
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Contains(t, w.Body.String(), "WEBHOOKER_ENVIRONMENT")
+}