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/common/stubs.rs

@@ -131,16 +131,6 @@ pub struct StubFileWritePort;
 
 #[async_trait]
 impl FileWritePort for StubFileWritePort {
-    async fn save_file(
-        &self,
-        _name: String,
-        _folder_id: Option<String>,
-        _content_type: String,
-        _content: Vec<u8>,
-    ) -> Result<File, DomainError> {
-        Ok(File::default())
-    }
-
     async fn save_file_from_temp(
         &self,
         _name: String,
@@ -177,14 +167,6 @@ impl FileWritePort for StubFileWritePort {
         Ok(())
     }
 
-    async fn update_file_content(
-        &self,
-        _file_id: &str,
-        _content: Vec<u8>,
-    ) -> Result<(), DomainError> {
-        Ok(())
-    }
-
     async fn update_file_content_from_temp(
         &self,
         _file_id: &str,
@@ -454,16 +436,6 @@ impl FileUploadUseCase for StubFileUploadUseCase {
         Ok(FileDto::default())
     }
 
-    async fn upload_file(
-        &self,
-        _name: String,
-        _folder_id: Option<String>,
-        _content_type: String,
-        _content: Vec<u8>,
-    ) -> Result<FileDto, DomainError> {
-        Ok(FileDto::default())
-    }
-
     async fn upload_file_from_path(
         &self,
         _name: String,