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
@@ -161,6 +163,7 @@ app = FastAPI(
     version="1.0.0",
     lifespan=lifespan,
 )
+STARTED_AT = str(int(time.time() * 1000))
 
 # No CORS middleware — same-origin via nginx (A1)
 
@@ -199,4 +202,4 @@ app.include_router(internal_router)
 
 @app.get("/health")
 async def health_check():
-    return {"status": "healthy"}
+    return {"status": "healthy", "version": os.environ.get("BUILD_VERSION", STARTED_AT)}