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/app/version.py

@@ -0,0 +1,5 @@
+"""Единый источник версии SAC (API, health, логи, OpenAPI)."""
+
+APP_NAME = "Security Alert Center"
+APP_VERSION = "0.2.0"
+APP_VERSION_LABEL = f"{APP_NAME} v.{APP_VERSION}"