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/fr.js

@@ -254,4 +254,15 @@ export default {
   "toast.deleted": "Supprimé",
   "toast.probing": "Sondage…",
   "toast.imported": "Config importée",
+  "dash.cache": "Cache sémantique",
+  "dash.cache.hitRate": "Taux de succès",
+  "dash.cache.hits": "Succès",
+  "dash.cache.saved": "Économisé",
+  "dash.cache.disabled": "Le cache sémantique est désactivé — activez-le dans les Paramètres.",
+  "settings.cache": "Cache sémantique",
+  "settings.cache.enabled": "Activer le cache sémantique",
+  "settings.cache.threshold": "Seuil de similarité",
+  "settings.cache.ttl": "TTL d'entrée",
+  "settings.cache.embed": "Modèle d'embeddings",
+  "settings.cache.note": "Les paramètres du cache prennent effet après un redémarrage.",
 };