narrow purge_stale_index_entries scan to indexed fields

[fabracht]

Context

PR #89 introduced purge_stale_index_entries in crates/mqdb-agent/src/database/query.rs. It scans the full idx/{entity}/ prefix and filters by ends_with("/{id}").

Proposed optimization

Ask the schema registry (or index manager) for the list of indexed fields on the entity, then scan only idx/{entity}/{field}/ for each. Typically a smaller cardinality than the entity-wide prefix scan.

Why

• Cheaper self-heal on entities with many indexed fields and large per-field key sets.
• Lets us drop the suffix-matching heuristic in favour of a more precise scan (still need ends_with because value may contain /, but the search space shrinks).

Out of scope

• The current full-prefix scan is correct and the self-heal path is rare. This is a perf optimisation, not a bug.