Add analytics for loaded Message Explorer results

### Problem\n\nMessage Explorer returns operationally useful metadata (Broker, queue, tag, producer/store host, timestamp, and size), but the page only renders individual rows. During incident investigation operators must manually estimate distribution, concentration, data volume, and missing routing metadata.\n\n### Proposed solution\n\nAdd a read-only result analysis drawer for the currently loaded query page. It provides Broker/queue/tag/host/hour distributions, size buckets, metadata completeness, explicit loaded-versus-server coverage, filtering, and CSV export.\n\nThe UI clearly discloses pagination and server scan truncation and does not issue extra message queries.

Start at the Message Explorer page and trace how the currently loaded query-page results are rendered. Define the read-only analysis drawer around those results, then verify distributions, size buckets, metadata completeness, pagination and scan-truncation disclosures, filtering, CSV export, and that no extra message queries are issued.

Written by the indexing model from the issue text.