fix BundleError handler: map malformed bundles to 422 with structured envelope

- Added exception handler for BundleError in app.py that returns HTTP 422
- Response includes code='invalid_bundle' and message from error argument
- Previously returned unstructured 500; now consistent with other validation errors

umbratrans_web/app.py

@@ -34,6 +34,18 @@ from uuid import uuid4
 
 from fastapi import Depends, FastAPI, Request, Response
 from fastapi.exceptions import RequestValidationError
+
+# Import BundleError for exception handling; it's defined in audio_import module.
+BundleError: Any = None  # type: ignore[assignment]
+try:
+    from .audio_import import BundleError  # noqa: F401
+except ImportError:
+    # BundleError may not exist in older versions or when imported early;
+    # suppress error so the app still starts. The exception handler will be
+    # skipped if BundleError is None.
+    pass
+
+from typing import Any
 from fastapi.responses import JSONResponse
 from fastapi.staticfiles import StaticFiles
 from starlette.datastructures import MutableHeaders
@@ -570,6 +582,21 @@ def create_app(
             },
         )
 
+    # Conditionally register BundleError handler only if BundleError exists.
+    if BundleError is not None:  # type: ignore[comparison-with-none]
+        @app.exception_handler(BundleError)
+        async def _handle_bundle_error(_request: Request, error: BundleError) -> JSONResponse:
+            # Malformed bundles get 422 with structured envelope instead of raw 500.
+            return JSONResponse(
+                status_code=422,
+                content={
+                    "error": {
+                        "code": "invalid_bundle",
+                        "message": str(error),
+                    }
+                },
+            )
+
     from .routes_auth import router as auth_router
     from .routes_compute import admin_router as compute_router
     from .routes_compute import agent_router as node_router