fix: read version from env var so it works inside Docker

backend/main.py

@@ -136,8 +136,9 @@ async def metrics():
 
 @app.get("/api/version")
 async def version():
-    version_file = Path(__file__).parent.parent / "VERSION"
-    return {"version": version_file.read_text().strip() if version_file.exists() else "unknown"}
+    import os
+
+    return {"version": os.environ.get("VERSION", "unknown")}
 
 
 frontend_dir = Path(__file__).parent / "frontend"