Add workflow engine scaffolding and APIs

[epuerta9]

Summary

• add workflow definition/run tables with accompanying sqlc queries and repositories to store versions, runs, and step history
• introduce workflow domain models, validation utilities, and service logic plus tests for creating workflows and starting runs
• expose workflow definition and run HTTP endpoints and wire them into the API router
• add Serverless Workflow translator and NATS-backed workflow engine options (including embedded JetStream) with runtime tests and optional auto-wiring via env vars

Testing

• go test ./internal/workflows
• go test ./internal/workflows/runtime
• go test ./internal/services -run ^TestWorkflowServiceCreateAndStartRun$ -count=1
• go test ./... (still fails: lighthouse integration and OpenAI plugin tests expect external services/keys)

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Codex Task