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/views/dashboard.js

@@ -108,6 +108,22 @@ export async function renderDashboard() {
         barList(breakdown, num)
       ),
 
+      // semantic cache
+      stats.cache && stats.cache.enabled
+        ? h(
+            "div",
+            { class: "card" },
+            h("div", { class: "card-head" }, h("h3", {}, t("dash.cache"))),
+            h(
+              "div",
+              { class: "kpis" },
+              kpi(pct(stats.cache.hit_rate), t("dash.cache.hitRate")),
+              kpi(num(stats.cache.hits), t("dash.cache.hits")),
+              kpi(money(stats.cache.saved_usd), t("dash.cache.saved"))
+            )
+          )
+        : null,
+
       // recent
       h(
         "div",