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/protocol.c

@@ -91,12 +91,12 @@ void send_data(int file_descriptor, Data *data) {
 }
 
 Data *receive_data(int file_descriptor) {
-  size_t size = 0;
+  unsigned long long size = 0;
   receive_n_data(file_descriptor, &size, sizeof(unsigned long long));
-  void *data = malloc(size);
-  receive_n_data(file_descriptor, data, size);
+  void *data = malloc((size_t)size);
+  receive_n_data(file_descriptor, data, (size_t)size);
   log_message(LOG_LEVEL_DEBUG, "Received %lld data", size);
-  return data_create(data, size);
+  return data_create(data, (size_t)size);
 }
 
 void send_int(int file_descriptor, int data) {