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/src/App.vue

@@ -1,7 +1,7 @@
 <template>
   <div class="layout">
     <nav v-if="showNav">
-      <span class="brand">SAC</span>
+      <span class="brand">{{ appTitle }}</span>
       <RouterLink to="/dashboard">Dashboard</RouterLink>
       <RouterLink to="/events">События</RouterLink>
       <RouterLink to="/problems">Проблемы</RouterLink>
@@ -16,6 +16,9 @@
 import { computed } from "vue";
 import { useRoute, useRouter } from "vue-router";
 import { clearToken, getToken } from "./api";
+import { APP_VERSION_LABEL } from "./version";
+
+const appTitle = APP_VERSION_LABEL;
 
 const route = useRoute();
 const router = useRouter();