Add BGM data directories to .gitignore

BGM files are user data and should not be tracked in git.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -5,9 +5,11 @@
 # Data directories
 data/downloads/
 data/processed/
+data/bgm/
 data/jobs.json
 backend/data/downloads/
 backend/data/processed/
+backend/data/bgm/
 backend/data/jobs.json
 backend/data/cookies/