TECH-5 · Orphan-task alerts + automatic redistribution

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

### Finding
When a volunteer disappears, their tasks are silently orphaned.

### Action
**Orphan-task alerts** + automatic redistribution. *(Product)*

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- **Priority:** 🟡 Medium
- **Dependencies:** INST-4 (#2494).
- **Epic:** #2503

The issue names no files, tests, or entry points. Start by reading dependency INST-4 (#2494) and epic #2503 to understand the existing task and volunteer lifecycle, then define acceptance criteria for detecting orphaned tasks, alerting about them, and redistributing them automatically.

索引モデルが issue の本文から書いたものです。