Heal Decay Bug

Restore rows soft-deleted by the pre-fix consolidator decay bug.

Before 2026-04-26, _decay_stale_rows halved confidence on any row where last_recalled_at IS NULL, treating freshly-inserted rows identically to 30-day-stale ones. Within a few 6h cycles new memory_dir / memory_md rows decayed below the 0.1 floor and were soft-deleted via expires_at = now().

This endpoint reverses that for rows that were unfairly decayed: rows where expires_at IS NOT NULL AND last_recalled_at IS NULL. A genuine 30-day-stale row would have last_recalled_at set (or created_at predating the threshold and last_recalled_at NULL, which we guard with a created_at floor).

Restores expires_at = NULL and resets confidence to the source's initial value (0.9 for memory_md, 0.5 for memory_dir, else 0.4). Idempotent — re-running on already-restored rows is a no-op.