refactor: extract receive_chunk_data and receive_manifest shared helpers

- receive_chunk_data(fd, config) -> Chunk*: shared receive/decompress/deserialize
  (eliminates ~25 lines of duplication between server.c and multiprocessing.c)
- receive_manifest(fd, config, next_status): moved from server.c to file.c,
  fixes missing 'Deleting files not in manifest...' log in multiprocessing.c
- Removes redundant manual free loop in multiprocessing.c manifest handling
  (array_list_create(free) destructor already handles this)
- server.c and multiprocessing.c: remove local chunk/manifest helpers,
  remove compression.h include (no longer needed)

src/shared/file.h

@@ -34,5 +34,6 @@ void file_metadata_destroy(void *metadata);
 bool to_disk(const char *path, const void *data, unsigned long long data_size);
 bool file_save_to_disk(const char *root_directory, File *file);
 File *receive_incremental_check(int fd, Config *config, bool *skipped);
+int receive_manifest(int fd, Config *config, int *next_status);
 
 #endif