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/.dashboard-opened

@@ -0,0 +1,2 @@
+380698
+1778526681407

.stoneforge/.gitignore

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+# Runtime data
+*.db
+*.db-journal
+*.db-wal
+*.db-shm
+daemon-state.json

.stoneforge/config.yaml

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+# 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

.stoneforge/sync/elements.jsonl

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+{"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"}