Dagu affected by unauthenticated RCE via inline DAG spec in default configuration

Summary

### Summary Dagu's default configuration ships with authentication disabled. The `POST /api/v2/dag-runs` endpoint accepts an inline YAML spec and executes its shell commands immediately with no credentials required — any dagu instance reachable over the network is fully compromised by default.

### Details `internal/service/app/config/loader.go:226` sets `AuthModeNone` as the default. With no auth mode configured, `internal/frontend/api/v2/handlers/api.go:520` returns `nil` from `requireExecute()` — all permission checks pass without a valid session.

The `POST /api/v2/dag-runs` endpoint accepts a `spec` field containing a full YAML DAG definition. The spec is parsed and the commands execute immediately on the host with no validation beyond YAML parsing.

### PoC ```bash curl -s -X POST http://TARGET:8080/api/v2/dag-runs \ -H "Content-Type: application/json" \ -d '{"name":"poc","spec":"steps:\n - name: rce\n command: id > /tmp/pwned\n"}' # Response: {"dagRunId":"<uuid>"} # /tmp/pwned contains: uid=1000(dagu) gid=1000(dagu) ``` Confirmed on `ghcr.io/dagu-org/dagu:latest` with no configuration changes.

### Impact Every dagu deployment using default settings — every Docker deployment, every install following the documentation, every instance without explicit `DAGU_AUTH_MODE` configuration — is fully compromised without credentials. An attacker with network access gets OS command execution as the dagu process user.