fix: delete_video - add file existence check + fix pre_chunks UUID cast

- unregister: check file exists before delete, return 200 with success:false if not found
- delete_video: cast pre_chunks.file_uuid parameter as UUID (::uuid)
- Added Phase 2 test script (10/10 endpoints passed)

src/api/server.rs

@@ -3525,6 +3525,22 @@ async fn unregister(
     }
 
     tracing::info!("[unregister] Unregistering file: {}", uuid);
+    
+    // Check if video exists first
+    match db.get_video_by_uuid(uuid).await {
+        Ok(Some(_)) => {},
+        Ok(None) => {
+            return Ok(Json(UnregisterResponse {
+                success: false,
+                file_uuid: uuid.to_string(),
+                message: "File not found".to_string(),
+            }));
+        }
+        Err(e) => {
+            return Err(StatusCode::INTERNAL_SERVER_ERROR);
+        }
+    }
+    
     match db.delete_video(uuid).await {
         Ok(_) => {
             let _ = state.mongo_cache.invalidate_videos_list().await;