Drop temperature param, unsupported on claude-sonnet-5

The live API rejected every request with "temperature is deprecated
for this model" once the model was updated to claude-sonnet-5.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app.py

@@ -618,7 +618,6 @@ MINIMAL MODE (user wants just the essentials):
     payload = {
         "model": "claude-sonnet-5",
         "max_tokens": 1000,
-        "temperature": 0.4,
         "system": system_prompt,
         "messages": [{"role": "user", "content": user_message}]
     }
@@ -635,7 +634,6 @@ async def chat(request: ChatRequest):
     payload = {
         "model": "claude-sonnet-5",
         "max_tokens": request.max_tokens,
-        "temperature": 0.5,
         "system": request.system,
         "messages": [{"role": m.role, "content": m.content} for m in request.messages]
     }