Database Persistence (System DB & History DB)

[kination]

Description

Add Ecto-backed persistent storage for DAG registry, run states, and task audit events.

Tasks

• Setup dual Ecto repos: Levicon.Repo.System and Levicon.Repo.History
• Create dags, runs, task_snapshots Ecto schemas and implement migrations
• Wire async DB writes from Task.Process state transitions (ETS primary, DB async)
• Implement startup recovery: rebuild ETS from task_snapshots on boot
• Create audit_events Ecto schema (append-only, no timestamps/0)
• Implement log writer called on every Task.Process state transition
• Phase 2: Enable TimescaleDB hypertable + retention policy migration