Add required @app .api endpoint to gradio.Server (empty Blocks exits on Space)

server.py

@@ -106,10 +106,21 @@ IMAGE_ONLY_MSG = (
 # gradio-SDK Space and wires up the queue + ZeroGPU GPU allocation. A plain
 # FastAPI + manual `uvicorn.run(7860)` collides with the Space's own gradio
 # server ("address already in use") — `gradio.Server` is the supported way to run
-# a custom frontend on a gradio Space. Present since gradio 6.17 (the pinned ver).
+# a custom frontend on a gradio Space.
 app = gr.Server(title="Recall")
 
 
+# A gradio.Server MUST register at least one `@app.api(...)` endpoint: launch()
+# builds its internal Blocks from these, and on a Space an *empty* Blocks doesn't
+# stay "running" (launch returns and the process exits → RUNTIME_ERROR). This is
+# also the gradio-native handle (`gradio_client` / the JS Client) onto the same
+# backend the custom frontend drives over plain JSON `/api/*` routes below.
+@app.api(name="health")
+def health() -> str:
+    """Liveness probe + the Server's required gradio endpoint."""
+    return "ok"
+
+
 # ---- serialization ---------------------------------------------------------
 
 def _card_out(card: dict | None) -> dict | None: