insight-chat: per-turn system_prompt override on chat continuation

Append mode: applied ephemerally — original system message restored
before persistence so re-opens see the baked persona. Amend mode:
override stays in place and becomes the new insight row's system
message. Pattern mirrors annotate_system_with_budget.

Adds system_prompt field on both ChatTurnHttpRequest and ChatTurnRequest;
plumbs through chat_turn and chat_turn_stream identically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ai/handlers.rs

@@ -640,6 +640,10 @@ pub struct ChatTurnHttpRequest {
     pub min_p: Option<f32>,
     #[serde(default)]
     pub max_iterations: Option<usize>,
+    /// Per-turn system-prompt override. Ephemeral in append mode,
+    /// persisted in amend mode. See ChatTurnRequest for semantics.
+    #[serde(default)]
+    pub system_prompt: Option<String>,
     #[serde(default)]
     pub amend: bool,
 }
@@ -695,6 +699,7 @@ pub async fn chat_turn_handler(
         top_k: request.top_k,
         min_p: request.min_p,
         max_iterations: request.max_iterations,
+        system_prompt: request.system_prompt.clone(),
         amend: request.amend,
     };
 
@@ -909,6 +914,7 @@ pub async fn chat_stream_handler(
         top_k: request.top_k,
         min_p: request.min_p,
         max_iterations: request.max_iterations,
+        system_prompt: request.system_prompt.clone(),
         amend: request.amend,
     };