Add comprehensive testing for preview clip and status handling

- Implement unit tests for PreviewClipRequest/PreviewStatusRequest serialization and deserialization.
- Add tests for PreviewDao (insert, update, batch retrieval, and status-based queries).
- Extend Actix-web integration tests for `/video/preview/status` endpoint scenarios.
- Introduce in-memory TestPreviewDao for mock database interactions.
- Update README with new config parameters for preview clips.

src/data/mod.rs

@@ -442,4 +442,54 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn test_preview_clip_request_deserialize() {
+        use super::PreviewClipRequest;
+        let json = r#"{"path":"photos/2024/video.mp4"}"#;
+        let req: PreviewClipRequest = serde_json::from_str(json).unwrap();
+        assert_eq!(req.path, "photos/2024/video.mp4");
+    }
+
+    #[test]
+    fn test_preview_status_request_deserialize() {
+        use super::PreviewStatusRequest;
+        let json = r#"{"paths":["a/one.mp4","b/two.mp4","c/three.mp4"]}"#;
+        let req: PreviewStatusRequest = serde_json::from_str(json).unwrap();
+        assert_eq!(req.paths.len(), 3);
+        assert_eq!(req.paths[0], "a/one.mp4");
+        assert_eq!(req.paths[2], "c/three.mp4");
+    }
+
+    #[test]
+    fn test_preview_status_response_serialize() {
+        use super::{PreviewStatusItem, PreviewStatusResponse};
+
+        let response = PreviewStatusResponse {
+            previews: vec![
+                PreviewStatusItem {
+                    path: "a/one.mp4".to_string(),
+                    status: "complete".to_string(),
+                    preview_url: Some("/video/preview?path=a%2Fone.mp4".to_string()),
+                },
+                PreviewStatusItem {
+                    path: "b/two.mp4".to_string(),
+                    status: "pending".to_string(),
+                    preview_url: None,
+                },
+            ],
+        };
+
+        let json = serde_json::to_value(&response).unwrap();
+        let previews = json["previews"].as_array().unwrap();
+        assert_eq!(previews.len(), 2);
+
+        // Complete item should have preview_url
+        assert_eq!(previews[0]["status"], "complete");
+        assert!(previews[0]["preview_url"].is_string());
+
+        // Pending item should not have preview_url (skip_serializing_if)
+        assert_eq!(previews[1]["status"], "pending");
+        assert!(previews[1].get("preview_url").is_none());
+    }
 }