feat: semantic (embedding-based) response cache

- cache.rs: cosine-similarity store (threshold, TTL, ring buffer), partitioned by
  routing signature; tracks hit rate and model cost saved
- pipeline: embed routing text, look up before the model call (route_source=cache on
  hit), insert successful responses; stats record cache hits
- config [cache] + admin settings card; dashboard shows hit-rate and savings
- embedding model resolved from the pool (or first kind="embedding")

web/js/i18n/de.js

@@ -254,4 +254,15 @@ export default {
   "toast.deleted": "Gelöscht",
   "toast.probing": "Prüfe…",
   "toast.imported": "Konfiguration importiert",
+  "dash.cache": "Semantischer Cache",
+  "dash.cache.hitRate": "Trefferquote",
+  "dash.cache.hits": "Treffer",
+  "dash.cache.saved": "Gespart",
+  "dash.cache.disabled": "Semantischer Cache ist deaktiviert — in den Einstellungen aktivieren.",
+  "settings.cache": "Semantischer Cache",
+  "settings.cache.enabled": "Semantischen Cache aktivieren",
+  "settings.cache.threshold": "Ähnlichkeitsschwelle",
+  "settings.cache.ttl": "Eintrags-TTL",
+  "settings.cache.embed": "Embedding-Modell",
+  "settings.cache.note": "Cache-Einstellungen werden nach einem Neustart wirksam.",
 };