Remove backup files and add pattern to .gitignore

Deleted stale backup files:
- frontends/cli/main.py_old
- src/stegasoo/dct_steganography.py_old

Added gitignore patterns for common backup extensions.

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

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

.gitignore

@@ -35,6 +35,12 @@ old_files/
 *.swp
 *.swo
 
+# Backup files
+*_old
+*_old.*
+*.bak
+*.orig
+
 # Testing
 .pytest_cache/
 .coverage