Make workflow and job gauges stateful and idempotent

[darthfork]

Parent issue: #35

Summary

Workflow/job gauges are currently mutated directly from event status values. This makes them vulnerable to duplicates, missed transitions, out-of-order events, and restarts.

Goals

• Track workflow and job states by stable identifiers.
• Apply transitions idempotently.
• Derive queued/in-progress/completed gauge values from tracked state.

Suggested implementation notes

• Use run_id and job id as stable state keys.
• Keep bounded in-memory state with TTL or eviction.
• Define behavior for restarts and incomplete history.

Acceptance criteria

• Duplicate and out-of-order events do not cause gauge drift.
• Gauge semantics are documented clearly.

[darthfork]

Fixed by #48.