Add JSON error logging for failed request deserialization

Configures a global JsonConfig error handler that logs the method, URI,
and parse error details at WARN level before returning the 400 response.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.rs

@@ -1212,6 +1212,21 @@ fn main() -> std::io::Result<()> {
                 .app_data::<Data<Mutex<Box<dyn PreviewDao>>>>(Data::new(Mutex::new(Box::new(
                     preview_dao,
                 ))))
+                .app_data(
+                    web::JsonConfig::default()
+                        .error_handler(|err, req| {
+                            let detail = err.to_string();
+                            log::warn!(
+                                "JSON parse error on {} {}: {}",
+                                req.method(),
+                                req.uri(),
+                                detail
+                            );
+                            let response = HttpResponse::BadRequest()
+                                .json(serde_json::json!({"error": detail}));
+                            actix_web::error::InternalError::from_response(err, response).into()
+                        }),
+                )
                 .app_data::<Data<InsightGenerator>>(Data::new(app_data.insight_generator.clone()))
                 .wrap(prometheus.clone())
         })