fix: dedupe host_silence problems under concurrent scans (0.4.11)

Treat acknowledged as active, dedupe by hostname, add PostgreSQL advisory lock and a unique index. Migration closes existing duplicate open problems.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/version.ts

@@ -1,4 +1,4 @@
 /** Fallback до загрузки /health; при релизе держите в sync с backend/app/version.py */
 export const APP_NAME = "Security Alert Center";
-export const APP_VERSION = "0.4.10";
+export const APP_VERSION = "0.4.11";
 export const APP_VERSION_LABEL = `${APP_NAME} v.${APP_VERSION}`;