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/multiprocessing.c

@@ -84,6 +84,10 @@ static void receive_chunk_enqueue(int file_descriptor,
   }
   Chunk *chunk = chunk_deserialize(data_to_process, context->config->use_metadata);
   data_destroy(data_to_process);
+  if (chunk == NULL) {
+    log_message(LOG_LEVEL_ERROR, "Failed to deserialize chunk, skipping");
+    return;
+  }
 
   for (int i = 0; i < chunk->element_count; i++) {
     File *file = chunk->items[i];