feat(ai): chat rewind + ollama metrics logging

Rewind: POST /insights/chat/rewind truncates training_messages at a
given rendered index, dropping the target message plus any preceding
tool-call scaffolding. The initial user prompt is protected.

Metrics: log prompt_eval_count/duration and eval_count/duration from
every Ollama chat response, rendered as tokens + ms + tok/s.

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

src/ai/handlers.rs

@@ -706,6 +706,63 @@ pub struct RenderedHistoryMessage {
     pub is_initial: bool,
 }
 
+#[derive(Debug, Deserialize)]
+pub struct ChatRewindHttpRequest {
+    pub file_path: String,
+    #[serde(default)]
+    pub library: Option<String>,
+    /// 0-based index into the rendered transcript. The message at this
+    /// index, and everything after it, is discarded. Must be > 0 — the
+    /// initial user message is protected.
+    pub discard_from_rendered_index: usize,
+}
+
+/// POST /insights/chat/rewind — truncate the stored conversation so the
+/// rendered message at `discard_from_rendered_index` (and everything after)
+/// is removed. Use when a user wants to retry a turn with a different
+/// prompt without prior replies poisoning context.
+#[post("/insights/chat/rewind")]
+pub async fn chat_rewind_handler(
+    _claims: Claims,
+    request: web::Json<ChatRewindHttpRequest>,
+    app_state: web::Data<AppState>,
+) -> impl Responder {
+    let library = match libraries::resolve_library_param(&app_state, request.library.as_deref()) {
+        Ok(Some(lib)) => lib,
+        Ok(None) => app_state.primary_library(),
+        Err(e) => {
+            return HttpResponse::BadRequest().json(serde_json::json!({
+                "error": format!("invalid library: {}", e)
+            }));
+        }
+    };
+
+    match app_state
+        .insight_chat
+        .rewind_history(
+            library.id,
+            &request.file_path,
+            request.discard_from_rendered_index,
+        )
+        .await
+    {
+        Ok(()) => HttpResponse::Ok().json(serde_json::json!({ "success": true })),
+        Err(e) => {
+            let msg = format!("{}", e);
+            log::error!("Chat rewind failed: {}", msg);
+            if msg.contains("no insight found") {
+                HttpResponse::NotFound().json(serde_json::json!({ "error": msg }))
+            } else if msg.contains("no chat history") {
+                HttpResponse::Conflict().json(serde_json::json!({ "error": msg }))
+            } else if msg.contains("cannot discard the initial") || msg.contains("out of range") {
+                HttpResponse::BadRequest().json(serde_json::json!({ "error": msg }))
+            } else {
+                HttpResponse::InternalServerError().json(serde_json::json!({ "error": msg }))
+            }
+        }
+    }
+}
+
 /// GET /insights/chat/history — return the rendered transcript for a photo.
 #[get("/insights/chat/history")]
 pub async fn chat_history_handler(