feat: Add API key authentication for backup tool

- Add get_api_key() and require_api_key() to auth.py
- Add generic key-value settings storage to SettingsStore
- Add test_api_key_auth to verify the implementation
- Uses secrets.compare_digest for timing-safe comparison
- Auto-generates API key on first use and stores in settings DB

.gitignore

@@ -47,3 +47,4 @@ frontend/dist/
 # Pi internal
 .pi-lens/
 .pi/
+/.stoneforge/.worktrees/