feat(04-02): apply locale fallback and missing-key logging

- Resolve pl-PL then parent then app.fallback_locale then the raw key
- Read towel.Locale from request context and default app.locale/fallback to en
- Log each missing key once outside production and never log parameters

Co-authored-by: Cursor <cursoragent@cursor.com>

examples/hello/hello_test.go

@@ -130,6 +130,16 @@ func TestHelloPluginResolvesLang(t *testing.T) {
 	if got := tr.GetIn("en", "golem15.hello::lang.greeting", nil); got != "Hello" {
 		t.Fatalf("greeting = %q", got)
 	}
+	if got := tr.GetIn("pl-PL", "golem15.hello::lang.greeting", nil); got != "Cześć" {
+		t.Fatalf("pl-PL = %q", got)
+	}
+	if got := tr.GetIn("de", "golem15.hello::lang.greeting", nil); got != "Hello" {
+		t.Fatalf("fallback en = %q", got)
+	}
+	const missing = "golem15.hello::lang.does.not.exist"
+	if got := tr.GetIn("pl-PL", missing, nil); got != missing {
+		t.Fatalf("raw key = %q", got)
+	}
 }
 
 func TestTypedItemRoute(t *testing.T) {