Add full DLQ backlog and aging portfolio analysis

### Problem\n\nThe DLQ page is paginated and supports per-group message operations, but operators cannot assess the full instance backlog, concentration, last-enqueue age, retry totals, or unavailable statistics without exporting and manually combining pages.\n\n### Proposed solution\n\nAdd an on-demand, read-only portfolio drawer that loads all DLQ groups in bounded pages and provides backlog totals, per-group shares, age buckets, status distribution, unavailable/unknown metadata counts, filters, and CSV export.\n\nThis feature does not read message bodies or invoke resend operations and is separate from per-group list performance and resend-result work.

Start with the DLQ page and its existing paginated group-loading path, then trace how per-group message operations are connected. Define the read-only portfolio drawer around bounded page loading, aggregation, filtering, and CSV export without reading message bodies or invoking resend operations. Done means the full-instance metrics and unavailable or unknown metadata states are visible and exportable.

Written by the indexing model from the issue text.