Ecto schema for persisted workflow instances.

Backed by the workflow_instances table with these fields:

• :id — workflow instance ID (business key format key::value)
• :workflow — workflow module name as a string
• :status — runtime status as a string
• :workflow_version — integer workflow version persisted separately from state
• :state — JSON-compatible serialized workflow state
• :inserted_at / :updated_at — Ecto timestamps