INST-7 · Onboarding milestones on the user

> Item derived from the fact-checking cycle analysis (**2026-07-03**). Production numbers (namespace `main`), unless noted.

### Finding (evidence)
Onboarding is only inferable from first activity; **39% of fact-checkers are one-off** (48 of 122) and the ~4–6 week ramp-up is only qualitative (N=30).

### Action
Milestones on the user:
- `onboarding.enabledAt`
- `onboarding.trainingCompletedAt`
- `onboarding.firstAssignmentAt`
- `onboarding.firstPublishedAt`
- `onboarding.isOneOff`
- `checksToSteadyState` (materialized)

### Impact
Time-to-productivity and retention become auditable metrics. Enables measuring **PROC-3**.

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- **Priority:** 🟡 Medium · **Effort:** S/M
- **Dependencies:** wave 5, alongside INST-6.
- **Epic:** #2489

Begin with epic #2489 and the wave 5/INST-6 dependency context. Then locate the user representation and the code paths that record onboarding activity; completion means the six onboarding fields and materialized checksToSteadyState are persisted and support auditing the stated metrics.

索引モデルが issue の本文から書いたものです。