fix(storage): handle cross-device rename and MKCOL on existing folders

- dedup_service: fall back to copy+delete when rename() fails with
  EXDEV (os error 18), which occurs when temp and blob dirs are on
  different filesystems
- NC webdav_handler: return 405 instead of 500 when MKCOL targets an
  existing folder (RFC 4918 §9.3.1)

src/infrastructure/services/dedup_service.rs

@@ -276,9 +276,20 @@ impl DedupService {
             })?;
 
             if let Err(e) = fs::rename(&temp_path, &blob_path).await {
-                // Another writer already placed the blob — discard ours
-                let _ = fs::remove_file(&temp_path).await;
-                tracing::debug!("Blob file already placed by concurrent writer: {}", e);
+                if e.raw_os_error() == Some(18) {
+                    // EXDEV: cross-device link — fall back to copy+delete
+                    fs::copy(&temp_path, &blob_path).await.map_err(|ce| {
+                        DomainError::internal_error(
+                            "Dedup",
+                            format!("Failed to copy temp blob cross-device: {}", ce),
+                        )
+                    })?;
+                    let _ = fs::remove_file(&temp_path).await;
+                } else {
+                    // Another writer already placed the blob — discard ours
+                    let _ = fs::remove_file(&temp_path).await;
+                    tracing::debug!("Blob file already placed by concurrent writer: {}", e);
+                }
             }
         }
 
@@ -365,8 +376,17 @@ impl DedupService {
             // dirs live on different filesystems (rare), this falls back to
             // copy+delete which is slower but still correct.
             if let Err(e) = fs::rename(source_path, &blob_path).await {
-                // Another writer may have placed the blob concurrently
-                if blob_path.exists() {
+                if e.raw_os_error() == Some(18) {
+                    // EXDEV: cross-device link — fall back to copy+delete
+                    fs::copy(source_path, &blob_path).await.map_err(|ce| {
+                        DomainError::internal_error(
+                            "Dedup",
+                            format!("Failed to copy file to blob store: {}", ce),
+                        )
+                    })?;
+                    let _ = fs::remove_file(source_path).await;
+                } else if blob_path.exists() {
+                    // Another writer may have placed the blob concurrently
                     let _ = fs::remove_file(source_path).await;
                     tracing::debug!("Blob file placed by concurrent writer: {}", e);
                 } else {

src/interfaces/nextcloud/webdav_handler.rs

@@ -572,15 +572,23 @@ async fn handle_mkcol(
         parent_id: Some(parent_folder.id.clone()),
     };
 
-    folder_service
-        .create_folder(dto)
-        .await
-        .map_err(|e| AppError::internal_error(format!("Failed to create folder: {}", e)))?;
-
-    Ok(Response::builder()
-        .status(StatusCode::CREATED)
-        .body(Body::empty())
-        .unwrap())
+    match folder_service.create_folder(dto).await {
+        Ok(_) => Ok(Response::builder()
+            .status(StatusCode::CREATED)
+            .body(Body::empty())
+            .unwrap()),
+        Err(e) if e.message.contains("already exists") || e.message.contains("Already Exists") => {
+            // RFC 4918 §9.3.1: MKCOL on existing resource → 405
+            Ok(Response::builder()
+                .status(StatusCode::METHOD_NOT_ALLOWED)
+                .body(Body::empty())
+                .unwrap())
+        }
+        Err(e) => Err(AppError::internal_error(format!(
+            "Failed to create folder: {}",
+            e
+        ))),
+    }
 }
 
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