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>

phrasebook/translator_test.go

@@ -1,13 +1,17 @@
 package phrasebook
 
 import (
+	"bytes"
+	"context"
 	"io/fs"
+	"log/slog"
 	"strings"
 	"testing"
 	"testing/fstest"
 
 	"git.golem15.com/golem15/summercms/backpack"
 	"git.golem15.com/golem15/summercms/compass"
+	"git.golem15.com/golem15/summercms/towel"
 )
 
 func TestTranslationSmoke(t *testing.T) {
@@ -189,3 +193,59 @@ func TestPluralSmoke(t *testing.T) {
 		}
 	})
 }
+
+func TestLocaleFallbackSmoke(t *testing.T) {
+	fsys := fstest.MapFS{
+		"lang/en/lang.yaml": {Data: []byte("greeting: Hello\nonlyEn: English only\n")},
+		"lang/pl/lang.yaml": {Data: []byte("greeting: Cześć\n")},
+	}
+	cat := NewCatalog()
+	if err := cat.Load("golem15.hello", fsys); err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	tr := NewTranslator(cat, Options{})
+	if tr.locale != "en" || tr.fallback != "en" {
+		t.Fatalf("defaults locale=%q fallback=%q, want en/en", tr.locale, tr.fallback)
+	}
+	if got := tr.GetIn("pl-PL", "golem15.hello::lang.greeting", nil); got != "Cześć" {
+		t.Fatalf("pl-PL parent = %q", got)
+	}
+	if got := tr.GetIn("pl", "golem15.hello::lang.onlyEn", nil); got != "English only" {
+		t.Fatalf("fallback en = %q", got)
+	}
+	const missing = "golem15.hello::lang.missing"
+	if got := tr.GetIn("pl-PL", missing, nil); got != missing {
+		t.Fatalf("raw key = %q", got)
+	}
+	ctx := towel.WithLocale(context.Background(), "pl")
+	if got := tr.Get(ctx, "golem15.hello::lang.greeting", nil); got != "Cześć" {
+		t.Fatalf("towel locale = %q", got)
+	}
+	if got := tr.Get(context.Background(), "golem15.hello::lang.greeting", nil); got != "Hello" {
+		t.Fatalf("app locale default = %q", got)
+	}
+
+	var buf bytes.Buffer
+	prev := slog.Default()
+	slog.SetDefault(slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{Level: slog.LevelWarn})))
+	defer slog.SetDefault(prev)
+
+	logged := "golem15.hello::lang.loggedOnce"
+	params := map[string]string{"secret": "nope"}
+	tr.GetIn("en", logged, params)
+	tr.GetIn("en", logged, params)
+	log := buf.String()
+	if strings.Count(log, logged) != 1 {
+		t.Fatalf("missing key should log once, got %q", log)
+	}
+	if strings.Contains(log, "secret") || strings.Contains(log, "nope") {
+		t.Fatalf("must not log parameters: %q", log)
+	}
+
+	buf.Reset()
+	prod := NewTranslator(cat, Options{Production: true})
+	prod.GetIn("en", "golem15.hello::lang.otherMissing", nil)
+	if buf.Len() != 0 {
+		t.Fatalf("production lookup must be silent, got %q", buf.String())
+	}
+}