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/style.css

@@ -36,6 +36,9 @@ nav .brand {
   font-weight: 700;
   color: #fff;
   margin-right: auto;
+  font-size: 0.95rem;
+  line-height: 1.3;
+  max-width: min(420px, 45vw);
 }
 
 table {
@@ -164,3 +167,11 @@ pre {
   flex-wrap: wrap;
   gap: 1rem;
 }
+
+.live-badge {
+  color: #6bcb77;
+  font-size: 0.85rem;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}