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/api/v1/router.py

@@ -1,6 +1,6 @@
 from fastapi import APIRouter
 
-from app.api.v1 import auth, dashboards, events, health, hosts, problems
+from app.api.v1 import auth, dashboards, events, health, hosts, problems, stream
 
 api_router = APIRouter()
 api_router.include_router(health.router)
@@ -9,3 +9,4 @@ api_router.include_router(events.router)
 api_router.include_router(hosts.router)
 api_router.include_router(problems.router)
 api_router.include_router(dashboards.router)
+api_router.include_router(stream.router)