perf: eliminate Vec<u8> buffer paths — all uploads now stream to disk

Issue #4 (HIGH): save_file(Vec<u8>) and update_file_content(Vec<u8>)
accepted up to 10 MB of contiguous memory per request. While the main
upload paths already used streaming, the WebDAV compat methods
(create_file, update_file) and the empty-file handler still used the
buffered path, creating a .to_vec() copy.

Changes:
- FileWritePort trait: remove save_file(Vec<u8>) and
  update_file_content(Vec<u8>) — only streaming variants remain
- FileUploadUseCase trait: remove upload_file(Vec<u8>)
- file_upload_service.rs: create_file() and update_file() now spool
  &[u8] to NamedTempFile + Sha256::digest, then delegate to streaming
  path (save_file_from_temp / update_file_streaming)
- file_handler.rs: empty file uploads use upload_file_streaming with
- FileBlobWriteRepository: remove save_file and update_file_content impls
- StubFileWritePort, StubFileUploadUseCase, MockFileRepository: remove
  corresponding dead method impls

Impact: impossible to accidentally use a buffered upload path. All
content goes through streaming with ~256 KB peak RAM. -166 LOC.

src/application/services/trash_service_test.rs

@@ -209,16 +209,6 @@ impl FileReadPort for MockFileRepository {
 
 #[async_trait]
 impl FileWritePort for MockFileRepository {
-    async fn save_file(
-        &self,
-        _name: String,
-        _folder_id: Option<String>,
-        _content_type: String,
-        _content: Vec<u8>,
-    ) -> std::result::Result<File, DomainError> {
-        unimplemented!()
-    }
-
     async fn save_file_from_temp(
         &self,
         _name: String,
@@ -251,14 +241,6 @@ impl FileWritePort for MockFileRepository {
         Ok(())
     }
 
-    async fn update_file_content(
-        &self,
-        _file_id: &str,
-        _content: Vec<u8>,
-    ) -> std::result::Result<(), DomainError> {
-        Ok(())
-    }
-
     async fn update_file_content_from_temp(
         &self,
         _file_id: &str,
@@ -514,10 +496,7 @@ mod tests {
         // Arrange
         let trashed_files = Arc::new(Mutex::new(HashMap::new()));
         let trashed_folders = Arc::new(Mutex::new(HashMap::new()));
-        let trash_repo = Arc::new(MockTrashRepository::new(
-            trashed_files.clone(),
-            trashed_folders.clone(),
-        ));
+        let trash_repo = Arc::new(MockTrashRepository::new(trashed_files.clone(), trashed_folders.clone()));
         let file_repo = Arc::new(MockFileRepository::new(trashed_files));
         let folder_repo = Arc::new(MockFolderRepository::new(trashed_folders));
 
@@ -588,10 +567,7 @@ mod tests {
         // Arrange
         let trashed_files = Arc::new(Mutex::new(HashMap::new()));
         let trashed_folders = Arc::new(Mutex::new(HashMap::new()));
-        let trash_repo = Arc::new(MockTrashRepository::new(
-            trashed_files.clone(),
-            trashed_folders.clone(),
-        ));
+        let trash_repo = Arc::new(MockTrashRepository::new(trashed_files.clone(), trashed_folders.clone()));
         let file_repo = Arc::new(MockFileRepository::new(trashed_files));
         let folder_repo = Arc::new(MockFolderRepository::new(trashed_folders));
 
@@ -653,10 +629,7 @@ mod tests {
         // Arrange
         let trashed_files = Arc::new(Mutex::new(HashMap::new()));
         let trashed_folders = Arc::new(Mutex::new(HashMap::new()));
-        let trash_repo = Arc::new(MockTrashRepository::new(
-            trashed_files.clone(),
-            trashed_folders.clone(),
-        ));
+        let trash_repo = Arc::new(MockTrashRepository::new(trashed_files.clone(), trashed_folders.clone()));
         let file_repo = Arc::new(MockFileRepository::new(trashed_files));
         let folder_repo = Arc::new(MockFolderRepository::new(trashed_folders));
 
@@ -723,10 +696,7 @@ mod tests {
         // Arrange
         let trashed_files = Arc::new(Mutex::new(HashMap::new()));
         let trashed_folders = Arc::new(Mutex::new(HashMap::new()));
-        let trash_repo = Arc::new(MockTrashRepository::new(
-            trashed_files.clone(),
-            trashed_folders.clone(),
-        ));
+        let trash_repo = Arc::new(MockTrashRepository::new(trashed_files.clone(), trashed_folders.clone()));
         let file_repo = Arc::new(MockFileRepository::new(trashed_files));
         let folder_repo = Arc::new(MockFolderRepository::new(trashed_folders));
 
@@ -792,10 +762,7 @@ mod tests {
         // Arrange
         let trashed_files = Arc::new(Mutex::new(HashMap::new()));
         let trashed_folders = Arc::new(Mutex::new(HashMap::new()));
-        let trash_repo = Arc::new(MockTrashRepository::new(
-            trashed_files.clone(),
-            trashed_folders.clone(),
-        ));
+        let trash_repo = Arc::new(MockTrashRepository::new(trashed_files.clone(), trashed_folders.clone()));
         let file_repo = Arc::new(MockFileRepository::new(trashed_files));
         let folder_repo = Arc::new(MockFolderRepository::new(trashed_folders));