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.go

@@ -3,6 +3,7 @@ package phrasebook
 import (
 	"context"
 	"fmt"
+	"log/slog"
 	"strconv"
 	"strings"
 	"sync"
@@ -55,6 +56,7 @@ type Translator struct {
 	locale     string
 	fallback   string
 	production bool
+	missing    sync.Map
 }
 
 // NewTranslator returns a translator that reads from cat. Empty locale or
@@ -122,7 +124,7 @@ func (t *Translator) GetIn(locale, key string, params map[string]string) string
 	if t == nil {
 		return key
 	}
-	e, ok := t.cat.lookup(locale, key)
+	e, _, ok := t.find(locale, key)
 	if !ok {
 		return key
 	}
@@ -143,20 +145,74 @@ func (t *Translator) ChoiceIn(locale, key string, count any, params map[string]s
 	if t == nil {
 		return key
 	}
-	e, ok := t.cat.lookup(locale, key)
+	e, found, ok := t.find(locale, key)
 	if !ok {
 		return key
 	}
 	text := e.text
 	switch {
 	case len(e.pipes) > 0:
-		text = selectPipe(e.pipes, locale, count)
+		text = selectPipe(e.pipes, found, count)
 	case e.plurals != nil:
-		text = selectMap(e.plurals, locale, count)
+		text = selectMap(e.plurals, found, count)
 	}
 	return interpolate(text, withCount(params, count))
 }
 
+func (t *Translator) find(locale, key string) (entry, string, bool) {
+	if t == nil {
+		return entry{}, locale, false
+	}
+	fallback := defaultFallback
+	if t.fallback != "" {
+		fallback = t.fallback
+	}
+	for _, step := range fallbackChain(locale, fallback) {
+		if e, ok := t.cat.lookup(step, key); ok {
+			return e, step, true
+		}
+	}
+	t.logMissing(key)
+	return entry{}, locale, false
+}
+
+func (t *Translator) logMissing(key string) {
+	if t.production {
+		return
+	}
+	if _, loaded := t.missing.LoadOrStore(key, struct{}{}); loaded {
+		return
+	}
+	slog.Warn("phrasebook: missing translation key", "key", key)
+}
+
+func fallbackChain(requested, fallback string) []string {
+	out := make([]string, 0, 4)
+	seen := make(map[string]struct{}, 4)
+	add := func(s string) {
+		s = strings.TrimSpace(s)
+		if s == "" {
+			return
+		}
+		if _, ok := seen[s]; ok {
+			return
+		}
+		seen[s] = struct{}{}
+		out = append(out, s)
+	}
+	add(requested)
+	if tag, err := language.Parse(requested); err == nil {
+		add(tag.String())
+		for p := tag.Parent(); !p.IsRoot(); p = p.Parent() {
+			add(p.String())
+		}
+	} else if i := strings.IndexAny(requested, "-_"); i > 0 {
+		add(requested[:i])
+	}
+	add(fallback)
+	return out
+}
+
 func selectMap(plurals map[string]string, locale string, count any) string {
 	cat, err := cldrCategory(locale, count)
 	if err != nil {