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/es.js

@@ -254,4 +254,15 @@ export default {
   "toast.deleted": "Eliminado",
   "toast.probing": "Sondeando…",
   "toast.imported": "Configuración importada",
+  "dash.cache": "Caché semántico",
+  "dash.cache.hitRate": "Aciertos",
+  "dash.cache.hits": "Aciertos",
+  "dash.cache.saved": "Ahorrado",
+  "dash.cache.disabled": "El caché semántico está desactivado — actívalo en Ajustes.",
+  "settings.cache": "Caché semántico",
+  "settings.cache.enabled": "Activar caché semántico",
+  "settings.cache.threshold": "Umbral de similitud",
+  "settings.cache.ttl": "TTL de entrada",
+  "settings.cache.embed": "Modelo de embeddings",
+  "settings.cache.note": "Los ajustes del caché se aplican tras reiniciar.",
 };