feat: v0.2.0 branding, healthz version, SSE dashboard live

Centralize APP_VERSION 0.2.0; /health and /healthz return version;
startup log line; UI title Security Alert Center v.0.2.0;
SSE /api/v1/stream/events for live dashboard counters.

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

frontend/vite.config.ts

@@ -8,6 +8,7 @@ export default defineConfig({
     proxy: {
       "/api": "http://127.0.0.1:8000",
       "/health": "http://127.0.0.1:8000",
+      "/healthz": "http://127.0.0.1:8000",
     },
   },
   build: {