feat(chat): stream model thinking over SSE and show it in a collapsible block

app/config.py

@@ -41,6 +41,11 @@ class Settings(BaseSettings):
     llm_api_key: str = ""
     llm_chat_model: str = "turbo"
     llm_embed_model: str = "embed"
+    #: Operator kill-switch for the ``thinking`` SSE events (phase 17,
+    #: ``BOR_STREAM_THINKING``; ``0``/``false`` → off). When off, thinking
+    #: pieces are still counted for the per-turn log line but never
+    #: emitted — the answer stream itself is unchanged.
+    stream_thinking: bool = True
 
     # --- RAG tuning ---
     embedding_dim: int = 768  # verified against aipi /v1 (embed model)