feat: add runnable simulator foundation

app/api/errors.py

@@ -0,0 +1,25 @@
+"""Base external error representation."""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+from fastapi import Request
+from fastapi.responses import JSONResponse
+
+logger = logging.getLogger(__name__)
+
+
+async def unhandled_exception_handler(request: Request, exc: Exception) -> JSONResponse:
+    """Log internal failures and return a stable non-FastAPI error envelope."""
+
+    logger.exception(
+        "unhandled request error",
+        extra={"request_id": getattr(request.state, "request_id", None), "path": request.url.path},
+    )
+    body: dict[str, Any] = {
+        "data": None,
+        "errors": {"internal": "internal server error"},
+    }
+    return JSONResponse(status_code=500, content=body)