feat(recipes): add recently opened sort with modal open tracking

Track recipe modal opens in a separate stats file (never touching recipe
JSON/EXIF), expose a fire-and-forget POST endpoint, and add an 'opened'
sort that hides never-opened recipes as a true recently-opened view.
Includes i18n for all locales and backend/frontend tests.

locales/de.json

@@ -920,7 +920,9 @@
                 "dateAsc": "Älteste",
                 "lorasCount": "LoRA-Anzahl",
                 "lorasCountDesc": "Meiste",
-                "lorasCountAsc": "Wenigste"
+                "lorasCountAsc": "Wenigste",
+                "opened": "Zuletzt geöffnet",
+                "openedDesc": "Zuletzt geöffnet"
             },
             "refresh": {
                 "title": "Rezeptliste aktualisieren",