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.

backend/tests/test_health.py

@@ -1,7 +1,14 @@
-"""Smoke tests — run with PostgreSQL or skip integration."""
+"""Health endpoint smoke tests."""
 
-import pytest
+from unittest.mock import MagicMock
+
+from app.api.v1.health import build_health_payload
+from app.version import APP_NAME, APP_VERSION
 
 
-def test_placeholder():
-    assert True
+def test_health_payload_includes_version():
+    db = MagicMock()
+    payload = build_health_payload(db)
+    assert payload["version"] == APP_VERSION
+    assert payload["name"] == APP_NAME
+    assert payload["database"] == "ok"