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/chunk.c

@@ -178,24 +178,5 @@ Data *chunk_compress(Chunk *chunk, int compression_level, bool use_metadata) {
   return compressed;
 }
 
-Chunk *chunk_decompress(Data *compressed_data, bool use_metadata) {
-  log_message(LOG_LEVEL_DEBUG, "Starting to decompress chunk");
-  Data *uncompressed_data = data_decompress(compressed_data);
-  if (uncompressed_data == NULL) {
-    log_message(LOG_LEVEL_ERROR, "Failed to decompress chunk data");
-    return NULL;
-  }
-
-  Chunk *chunk = chunk_deserialize(uncompressed_data, use_metadata);
-  if (chunk == NULL) {
-    log_message(LOG_LEVEL_ERROR, "Failed to deserialize chunk data");
-    data_destroy(uncompressed_data);
-    return NULL;
-  }
-
-  data_destroy(uncompressed_data);
-  log_message(LOG_LEVEL_DEBUG, "Chunk successfully decompressed");
-  return chunk;
-}