Data model for workflows

Part of: #276 (closed)

We need a DB model to track known workflows. Storing the state in the DB allows for visualization of work requests.

The design for this landed in !630 (merged), and is visible at https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/workflows.html#database-models. Leaving the logic changes aside from now, this should be straightforward to implement at this point.

Roadmap:

• Implement WorkflowTemplate (!734 (merged))
• Add WorkRequest.internal_collection (!764 (merged), !773 (merged))
• Add WorkRequest.expiration_delay (#365 (closed))
• Use pydantic for workflow_data (!756 (merged))
• Use pydantic for event_reactions (#355 (closed))