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/config.yaml

@@ -0,0 +1,20 @@
+# Stoneforge Configuration
+
+database: stoneforge.db
+sync:
+  auto_export: true
+  elements_file: elements.jsonl
+  dependencies_file: dependencies.jsonl
+playbooks:
+  paths:
+    - playbooks
+identity:
+  mode: soft
+merge:
+  auto_merge: true
+  target_branch: stoneforge/review
+  require_approval: false
+workflow:
+  preset: review
+agents:
+  permission_model: unrestricted