Add `filtergraph_applied` to --json success output

[swperb]

Add the resolved filter chain to the success JSON so pipelines can log/replay/debug:

{"ok":true,"output":"out.png","width":800,"height":600,"format":"png","bytes":34122,"filtergraph_applied":"scale=800:-1,grayscale"}

Where

src/main.c success block (and the batch per-file result). Emit the -vf graph string (empty string if none) via json_str. Keep the human output unchanged.

Done when

• field present in single + batch --json output; a tests/run.sh schema check asserts it
  Difficulty: easy.