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

tests/test_compression.c

@@ -86,7 +86,10 @@ static void test_chunk_compress_decompress_roundtrip() {
   Data *compressed = chunk_compress(chunk, 3, false);
   EXPECT_NOT_NULL(compressed);
 
-  Chunk *decompressed_chunk = chunk_decompress(compressed, false);
+  Data *decompressed_data = data_decompress(compressed);
+  EXPECT_NOT_NULL(decompressed_data);
+
+  Chunk *decompressed_chunk = chunk_deserialize(decompressed_data, false);
   EXPECT_NOT_NULL(decompressed_chunk);
   EXPECT_EQ_INT(decompressed_chunk->element_count, 2);
 
@@ -100,6 +103,7 @@ static void test_chunk_compress_decompress_roundtrip() {
 
   chunk_destroy(chunk);
   data_destroy(compressed);
+  data_destroy(decompressed_data);
   chunk_destroy(decompressed_chunk);
 
   unlink(path1);