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/sync/elements.jsonl

@@ -0,0 +1 @@
+{"id":"el-2rt3","type":"entity","createdAt":"2026-05-11T19:11:16.359Z","updatedAt":"2026-05-11T19:20:17.978Z","createdBy":"el-0000","tags":[],"metadata":{"agent":{"agentRole":"director","sessionStatus":"idle","maxConcurrentTasks":1,"channelId":"el-4cua","provider":"opencode","model":"kimi-for-coding/k2p6"}},"name":"director","entityType":"agent"}