Phase 1+2: bug fixes and dead code cleanup

Phase 1 — Bugs:
- receive_data: fix unsigned long long vs size_t mismatch
- chunk_deserialize: add NULL check in multiprocessing.c and server.c
- file_content_to_buffer: add missing fclose on error path
- send_files_multithreaded: propagate sender thread result
- server: add SIGPIPE handler
- to_disk: check fwrite return value
- scanner: use S_ISDIR instead of !S_ISREG
- scanner: free cur_path before early return

Phase 2 — Cleanup:
- Remove unused chunk_decompress function
- Remove unused array_list_clear function
- Remove unused num_connections config field
- Remove duplicate FILE_METADATA_WIRE_SIZE macro
- Update tests for removed APIs

src/shared/array_list.c

@@ -35,14 +35,6 @@ void array_list_delete(ArrayList *array_list) {
   free(array_list);
 }
 
-void array_list_clear(ArrayList *array_list) {
-  if (array_list == NULL)
-    return;
-  for (int i = 0; i < array_list->size; i++)
-    array_list->items[i] = NULL;
-  array_list->size = 0;
-}
-
 void array_list_extend(ArrayList *array_list) {
   if (array_list == NULL)
     return;