TECH-1 · review_time_minutes + accuracy_binary as computed DB fields

> Product/tech item from the dimensional diagnosis (**2026-07-03, v2**, section C).

### Finding
`review_time_minutes` and `accuracy_binary` exist only in offline analysis today.

### Action
Compute them as durable DB fields, extending `cycleMetrics` (INST-2 #2492). *(Product)*

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- **Priority:** 🟡 Medium
- **Dependencies:** INST-2 (#2492).
- **Epic:** #2503

Start by reading the dimensional diagnosis section C and the implementation for cycleMetrics in INST-2 (#2492), then locate where the offline analysis currently derives review_time_minutes and accuracy_binary. Confirm how computed values are persisted as durable DB fields and verify both fields are available through the extended cycleMetrics result.

索引モデルが issue の本文から書いたものです。