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/client/client_cli.c

@@ -46,7 +46,7 @@ int main(int argc, char *argv[]) {
   }
 
   Config *config = config_create(str_dup("1.0.0"), NULL, NULL,
-                                 save_to_disk, false, false, false, false, 5, 20, false);
+                                 save_to_disk, false, false, false, false, 5, false);
 
   int positional_args[2];
   int positional_count = 0;