self-heal stale index entries on list NotFound

[fabracht]

Summary

• list_from_index_ids now purges stale idx/{entity}/.../{id} entries whose data row is missing, instead of just logging a WARN each call. Race-safe: re-checks the row is absent before deleting.
• Atomic row+index delete (95c138c) already prevents new stale entries; this self-heals pre-existing ones and any from paths outside the agent crate.

Test plan

• cargo make dev (format + clippy + 748+ tests)
• list_purges_stale_index_entry_for_missing_row — manually injected stale entry is purged on next indexed list
• purge_skips_when_row_was_recreated_before_heal — race guard leaves entries alone when row exists