fix(server): return {message} body for request validation errors

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/app.py

@@ -2,6 +2,7 @@
 import os
 
 from fastapi import Depends, FastAPI, Header, HTTPException, Query
+from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
 
 from . import actions, jobs, mf
@@ -23,6 +24,11 @@ async def _http_exc(_req, exc: HTTPException):
     return JSONResponse(status_code=exc.status_code, content={"message": exc.detail})
 
 
+@app.exception_handler(RequestValidationError)
+async def _validation_exc(_req, exc: RequestValidationError):
+    return JSONResponse(status_code=422, content={"message": "Invalid or missing request parameters."})
+
+
 @app.get("/health")
 def health():
     return {"status": "ok"}