AI: add enable_thinking reasoning toggle plumbed to llama.cpp

New optional SamplingOverride forwarded to llama-server as
chat_template_kwargs.enable_thinking (gates Qwen3-style reasoning
blocks). None leaves the template default; other backends ignore it.
Wired through the agentic-insight and chat-turn request bodies/handlers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/reels/script.rs

@@ -309,6 +309,7 @@ pub async fn generate_script_agentic(
                 top_p: None,
                 top_k: None,
                 min_p: None,
+                enable_thinking: None,
             },
         )
         .await