fix: extend OrgStructureCache TTL and add graceful shutdown cleanup

- Extended OrgStructureCache TTL from 10 to 30 minutes (reduce cleanup frequency)
- Added orgStructureCache.destroy() in graceful shutdown handler
- Updated documentation to reflect changes

Co-Authored-By: Claude (glm-4.7) <noreply@anthropic.com>

src/app.ts

@@ -146,6 +146,10 @@ async function main() {
       logMemoryStore.destroy();
       console.log("[APP] LogMemoryStore destroyed");
 
+      // Destroy OrgStructureCache
+      orgStructureCache.destroy();
+      console.log("[APP] OrgStructureCache destroyed");
+
       clearTimeout(shutdownTimeout);
       console.log("[APP] Graceful shutdown completed");
       process.exit(0);