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

.stoneforge/.gitignore

@@ -0,0 +1,6 @@
+# Runtime data
+*.db
+*.db-journal
+*.db-wal
+*.db-shm
+daemon-state.json