Fix chat hang: stop sending num_ctx to the LLM gateway

num_ctx makes Ollama reload the model at a different context size (an
unload+load). Against a resident qwen3:32b at ctx 32768, our request asked for
8192 — forcing a 29GB reload on every call, which stalls indefinitely whenever
the backend's eviction path is unhealthy.

A/B against the gateway, identical requests otherwise:
  stream, no num_ctx    -> 200 in 0.31s
  stream, num_ctx=8192  -> http 000, hangs 60s+

It was also counterproductive: it shrank the context the model already served.

backend/src/spicebook/chat/llm.py

@@ -97,7 +97,10 @@ async def chat_completion_stream(
             "messages": messages,
             "temperature": 0.3,
             "max_tokens": settings.chat_max_tokens,
-            "num_ctx": settings.chat_max_tokens,
+            # No num_ctx: it makes Ollama reload the model at a different context
+            # size, which is an unload+load. That stalls whenever the backend's
+            # eviction path is unhealthy, and it would *shrink* the context the
+            # resident model already serves. Let the gateway pick the context.
             "stream": True,
         },
     ) as resp: