chore: gitignore local corpus data

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -12,3 +12,6 @@ backend/.venv/
 # contains live API keys — never commit; reference only
 pathfinder_setup_fns.py
 astroparse.zip
+
+# local corpus data (built by scripts/build_corpus.py)
+backend/data/