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")

src/config.rs

@@ -31,6 +31,8 @@ pub struct Config {
     pub admin: AdminCfg,
     #[serde(default)]
     pub stats: StatsCfg,
+    #[serde(default)]
+    pub cache: CacheCfg,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize)]
@@ -320,6 +322,46 @@ fn default_stats_retention() -> String {
     "7d".into()
 }
 
+/// Semantic (embedding-based) response cache — returns a cached answer for prompts
+/// that are semantically near a previous one, skipping the (expensive) model call.
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct CacheCfg {
+    #[serde(default)]
+    pub enabled: bool,
+    /// Cosine-similarity threshold for a hit (0..1).
+    #[serde(default = "default_semcache_threshold")]
+    pub threshold: f32,
+    /// Entry time-to-live, e.g. `1h`.
+    #[serde(default = "default_semcache_ttl")]
+    pub ttl: String,
+    /// Maximum number of cached entries (ring buffer).
+    #[serde(default = "default_semcache_max")]
+    pub max_entries: usize,
+    /// Embedding model id (defaults to the first `kind = "embedding"` model).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub embed_model: Option<String>,
+}
+impl Default for CacheCfg {
+    fn default() -> Self {
+        Self {
+            enabled: false,
+            threshold: default_semcache_threshold(),
+            ttl: default_semcache_ttl(),
+            max_entries: default_semcache_max(),
+            embed_model: None,
+        }
+    }
+}
+fn default_semcache_threshold() -> f32 {
+    0.92
+}
+fn default_semcache_ttl() -> String {
+    "1h".into()
+}
+fn default_semcache_max() -> usize {
+    1000
+}
+
 impl Config {
     /// Load and validate the config from a file.
     pub fn load(path: &str) -> anyhow::Result<Self> {