fix(zip): stream ZIP to temp file instead of loading entire archive into RAM

Solution C - Hybrid temp-file streaming:
- ZipPort trait now returns NamedTempFile instead of Vec<u8>
- ZipService writes to a temp file via ZipWriter<std::fs::File> (O(1) RAM)
- Files are read in 64KB stream chunks via get_file_stream() instead of get_file_content()
- HTTP response streams the temp file via ReaderStream (never loads full ZIP in memory)
- Temp file auto-deleted on drop after response completes
- Removed dead imports (HeaderName, HeaderValue, Cursor, Read)

src/common/stubs.rs

@@ -50,7 +50,7 @@ impl ZipPort for StubZipPort {
         &self,
         _folder_id: &str,
         _folder_name: &str,
-    ) -> Result<Vec<u8>, DomainError> {
+    ) -> Result<tempfile::NamedTempFile, DomainError> {
         Err(DomainError::internal_error(
             "ZipService",
             "ZipService not initialized",