Add first-class workflow runs and board UI visibility

[skylarbpayne]

This PR includes the first usable workflow vertical slice.

Backend

• add workflow definitions, runs, steps, and step events
• add workflow API routes
• add board snapshot workflow summaries
• add migration + tests

Frontend

• render workflow runs in board UI from snapshot data
• add workflow runs panel + tests

Verification

• targeted backend tests passed
• targeted frontend tests passed
• frontend production build passed