Fix 422 on text-only conversation turns

UploadFile | None = File(None) with from __future__ import annotations
caused FastAPI to still treat audio as required when omitted.
Changed to Optional[UploadFile] = None (no File() wrapper) so the
field is genuinely optional for text-input turns.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,11 @@ Follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/) · versioned wi
 
 ### Fixed
 
+- **Text-input turns returned 422** — `UploadFile | None = File(None)` with
+  `from __future__ import annotations` caused FastAPI to treat `audio` as a
+  required field even when omitted. Changed to `Optional[UploadFile] = None`
+  (no `File()` wrapper) so the field is genuinely optional for text-only turns.
+
 - **Conversation input bar hidden when mic unavailable** — the mic-blocked
   warning box was inserted before `conv-chat-window` inside the flex column,
   pushing the input bar off-screen. Fixed by prepending the warning inside

routes/conversation.py

@@ -11,7 +11,8 @@ import wave
 from pathlib import Path
 
 import requests
-from fastapi import APIRouter, File, Form, HTTPException, Request, UploadFile
+from typing import Optional
+from fastapi import APIRouter, Form, HTTPException, Request, UploadFile
 from fastapi.responses import Response, StreamingResponse
 
 from core.config import _load_settings, _save_settings, _clean_preview_backend
@@ -659,7 +660,7 @@ async def conversation_llm_models(url: str = ""):
 
 @router.post("/api/conversation/turn")
 async def conversation_turn(
-    audio:         UploadFile | None = File(None),
+    audio:         Optional[UploadFile] = None,
     text:          str = Form(""),
     stt_backend:   str = Form("configured"),
     llm_url:       str = Form(""),