perf(gateway): 为 dashboard 接口添加短时响应缓存

新增 DashboardResponseCache，对 stats/daily_stats/provider_status
三个接口按用户维度缓存 15-30 秒，减少重复数据库查询。

apps/aether-gateway/src/cache/dashboard_response.rs

@@ -0,0 +1,28 @@
+use std::time::Duration;
+
+use aether_cache::ExpiringMap;
+
+const MAX_ENTRIES: usize = 256;
+
+#[derive(Debug)]
+pub(crate) struct DashboardResponseCache {
+    entries: ExpiringMap<String, Vec<u8>>,
+}
+
+impl Default for DashboardResponseCache {
+    fn default() -> Self {
+        Self {
+            entries: ExpiringMap::new(),
+        }
+    }
+}
+
+impl DashboardResponseCache {
+    pub(crate) fn get(&self, key: &str, ttl: Duration) -> Option<Vec<u8>> {
+        self.entries.get_fresh(&key.to_string(), ttl)
+    }
+
+    pub(crate) fn insert(&self, key: String, value: Vec<u8>, ttl: Duration) {
+        self.entries.insert(key, value, ttl, MAX_ENTRIES);
+    }
+}