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/en.js

@@ -255,4 +255,15 @@ export default {
   "toast.deleted": "Deleted",
   "toast.probing": "Probing…",
   "toast.imported": "Config imported",
+  "dash.cache": "Semantic cache",
+  "dash.cache.hitRate": "Hit rate",
+  "dash.cache.hits": "Hits",
+  "dash.cache.saved": "Saved",
+  "dash.cache.disabled": "Semantic cache is disabled — enable it in Settings.",
+  "settings.cache": "Semantic cache",
+  "settings.cache.enabled": "Enable semantic cache",
+  "settings.cache.threshold": "Similarity threshold",
+  "settings.cache.ttl": "Entry TTL",
+  "settings.cache.embed": "Embedding model",
+  "settings.cache.note": "Cache settings take effect after a restart.",
 };