Add update-available banner + versioned /health endpoint

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/main.py

@@ -1,4 +1,6 @@
 import logging
+import os
+import time
 from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
@@ -57,10 +59,11 @@ app = FastAPI(
     version="1.0.0",
     lifespan=lifespan,
 )
+STARTED_AT = str(int(time.time() * 1000))
 
 app.include_router(kds_api.router, prefix="/api/kds", tags=["KDS"])
 
 
 @app.get("/health")
 async def health_check():
-    return {"status": "healthy"}
+    return {"status": "healthy", "version": os.environ.get("BUILD_VERSION", STARTED_AT)}