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/ports/storage_ports.rs

@@ -197,15 +197,6 @@ pub struct CopyFolderTreeResult {
 /// and deferred registration for the write-behind cache.
 #[async_trait]
 pub trait FileWritePort: Send + Sync + 'static {
-    /// Saves a new file from bytes.
-    async fn save_file(
-        &self,
-        name: String,
-        folder_id: Option<String>,
-        content_type: String,
-        content: Vec<u8>,
-    ) -> Result<File, DomainError>;
-
     /// Streaming upload — saves a file from a temp file already on disk.
     ///
     /// When `pre_computed_hash` is provided, the dedup service skips the
@@ -233,10 +224,6 @@ pub trait FileWritePort: Send + Sync + 'static {
     /// Deletes a file.
     async fn delete_file(&self, id: &str) -> Result<(), DomainError>;
 
-    /// Updates the content of an existing file.
-    async fn update_file_content(&self, file_id: &str, content: Vec<u8>)
-    -> Result<(), DomainError>;
-
     /// Streaming update — replaces file content from a temp file on disk.
     ///
     /// When `pre_computed_hash` is provided, the dedup service skips the