fix(i18n): publish the translation bundle only once it is complete

InitLocalizer filled the shared bundle in place, and the Telegram bot
runs it again while the panel is serving, so a request could read the
bundle's maps while they were being written and the process died on a
concurrent map access. Build the bundle privately and publish it, and
the bot and web localizers, through atomic pointers; the lazy load in
the middleware publishes only if nothing else did first.

internal/web/locale/locale_test.go

@@ -0,0 +1,48 @@
+package locale
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"testing"
+	"testing/fstest"
+
+	"github.com/gin-gonic/gin"
+)
+
+type fixedLang string
+
+func (l fixedLang) GetTgLang() (string, error) { return string(l), nil }
+
+// The Telegram bot re-runs InitLocalizer while the panel serves requests; a request must
+// never read the bundle while it is being filled (a concurrent map access kills the process).
+func TestInitLocalizerWhileServingRequests(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	translations := fstest.MapFS{"translation/en_US.json": {Data: []byte(`{"hello": "Hello"}`)}}
+	engine := gin.New()
+	engine.Use(LocalizerMiddleware())
+	engine.GET("/", func(c *gin.Context) { c.String(http.StatusOK, I18nForLang("en-US", "hello")) })
+
+	var wg sync.WaitGroup
+	wg.Go(func() {
+		for range 200 {
+			if err := InitLocalizer(translations, fixedLang("en-US")); err != nil {
+				t.Errorf("InitLocalizer: %v", err)
+				return
+			}
+		}
+	})
+	for range 4 {
+		wg.Go(func() {
+			for range 200 {
+				engine.ServeHTTP(httptest.NewRecorder(), httptest.NewRequest(http.MethodGet, "/", nil))
+			}
+		})
+	}
+	wg.Wait()
+	rec := httptest.NewRecorder()
+	engine.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/", nil))
+	if rec.Body.String() != "Hello" {
+		t.Fatalf("after re-initialisation the bundle answers %q, want Hello", rec.Body.String())
+	}
+}