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

.fusion/memory/DREAMS.md

@@ -0,0 +1,3 @@
+# Memory Dreams
+
+<!-- Periodic synthesized patterns from daily notes. Promote durable lessons to MEMORY.md. -->