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.h

@@ -18,7 +18,5 @@ void chunk_destroy(void *chunk);
 Data *chunk_serialize(Chunk *chunk, bool use_metadata);
 Chunk *chunk_deserialize(Data *data, bool use_metadata);
 Data *chunk_compress(Chunk *chunk, int compression_level, bool use_metadata);
-Chunk *chunk_decompress(Data *compressed_data, bool use_metadata);
-
 
 #endif