Provide dummy submission capability for testing

Implement a dummy submission feature that creates a placeholder submission for testing and demonstration purposes. Use case: QA engineers and developers can generate a predictable, non-production submission to verify workflows, UI rendering, and integrations without affecting real data. Expected behavior: creating a dummy submission produces a record with predefined sample fields, is distinguishable from real submissions (e.g., a 'dummy' flag), appears in submission lists and logs, does not trigger production notifications or external side effects, and can be easily removed or expired.