Remove instance/ from tracking, fix ruff lint errors

Security:
- Remove instance/.secret_key and instance/stegasoo.db from git
- Add instance/ to .gitignore (was only ignoring frontends/web/instance/)

Lint fixes:
- Remove unused imports in temp_storage.py (os, shutil)
- Sort imports and fix f-string placeholders in cli.py

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

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

frontends/web/temp_storage.py

@@ -14,8 +14,6 @@ It does NOT touch instance/ (auth database) or any other directories.
 """
 
 import json
-import os
-import shutil
 import time
 from pathlib import Path
 from threading import Lock