perf: stream_files_in_subtree — replace Vec<File> with async Stream

Replace list_files_in_subtree (fetch_all → Vec) with stream_files_in_subtree
that returns a Pin<Box<dyn Stream<Item = Result<File/FileDto>>>> backed by a
PostgreSQL cursor via sqlx::fetch().

Changes:
- FileReadPort::stream_files_in_subtree() returns streaming cursor (no default)
- FileRetrievalUseCase::stream_files_in_subtree() maps File→FileDto on the fly
- FileBlobReadRepository: async_stream::try_stream! + sqlx::fetch() cursor
- batch_operations: consume stream into HashMap incrementally
- zip_service: consume stream into HashMap incrementally
- All stubs/mocks updated (return empty stream)

Eliminates:
- Double allocation: Vec<(9-tuple)> + Vec<File> materialized simultaneously
- Unbounded RAM proportional to subtree size (was ~500 bytes × N files)
- Latency: callers blocked until last row fetched from PG

RAM is now O(folders) for the HashMap, not O(files).

src/application/services/share_service.rs

@@ -505,6 +505,16 @@ mod tests {
         ) -> Result<usize, DomainError> {
             Ok(0)
         }
+
+        async fn stream_files_in_subtree(
+            &self,
+            _folder_id: &str,
+        ) -> Result<
+            std::pin::Pin<Box<dyn futures::Stream<Item = Result<crate::domain::entities::file::File, DomainError>> + Send>>,
+            DomainError,
+        > {
+            Ok(Box::pin(futures::stream::empty()))
+        }
     }
 
     #[async_trait]