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/file_ports.rs

@@ -15,13 +15,18 @@ use crate::common::errors::DomainError;
 
 /// Primary port for file upload operations.
 ///
-/// All upload paths converge on streaming-to-disk:
+/// **All upload paths converge on streaming-to-disk** — no method accepts
+/// `Vec<u8>` for content.  Even `create_file` / `update_file` (WebDAV
+/// helpers that receive `&[u8]`) spool to a temp file internally so that
+/// peak RAM stays at ~256 KB regardless of file size.
+///
 /// - Normal uploads: handler spools multipart to temp file → `upload_file_streaming`
-/// - WebDAV PUT: small in-memory buffer → `upload_file`
 /// - Chunked uploads: chunks already on disk → `upload_file_from_path`
+/// - WebDAV PUT (new): handler streams to temp file → `update_file_streaming`
+/// - WebDAV PUT (small/compat): `create_file` / `update_file` spool internally
 #[async_trait]
 pub trait FileUploadUseCase: Send + Sync + 'static {
-    /// Upload from a temp file already on disk (true streaming, ~64 KB RAM).
+    /// Upload from a temp file already on disk (true streaming, ~256 KB RAM).
     ///
     /// When `pre_computed_hash` is `Some`, the blob store skips the hash
     /// re-read — the handler already computed it during the multipart spool.
@@ -35,19 +40,6 @@ pub trait FileUploadUseCase: Send + Sync + 'static {
         pre_computed_hash: Option<String>,
     ) -> Result<FileDto, DomainError>;
 
-    /// Upload from in-memory bytes (for small payloads: WebDAV, empty files).
-    ///
-    /// Only used for WebDAV PUT and empty files where the content is already
-    /// buffered by the protocol handler. For normal uploads, prefer
-    /// `upload_file_streaming`.
-    async fn upload_file(
-        &self,
-        name: String,
-        folder_id: Option<String>,
-        content_type: String,
-        content: Vec<u8>,
-    ) -> Result<FileDto, DomainError>;
-
     /// Upload from a file already assembled on disk (chunked uploads).
     ///
     /// Same as `upload_file_streaming` but with a separate name for clarity.