Add cross-page system alert incident timeline

### Problem

System Alerts exposes related events only from an individual row. Operators cannot review the complete event history as incidents, distinguish active and resolved sequences across pages, or summarize duration, acknowledgements, and notification suppression without opening records one at a time.

### Proposed change

Add a read-only incident timeline explorer that:

- loads alert history through the existing paginated endpoint, capped at the latest 10,000 events with explicit truncation disclosure;
- correlates events by server fingerprint first, then by rule + instance + complete labels only when a fingerprint is absent;
- keeps events without sufficient correlation evidence isolated;
- derives incident state from the latest transition and reports duration, severity, event, acknowledgement, and suppression counts;
- provides filters, expandable event timelines, CSV export, Chinese/English copy, and focused tests.

The explorer must not acknowledge, clear, silence, or otherwise mutate alerts.

Start by tracing the existing paginated System Alerts history endpoint and the individual-row related-events view. Done means a read-only explorer correlates and summarizes up to 10,000 events with truncation disclosure, filters and CSV export, Chinese/English copy, isolated uncorrelated events, and focused tests without mutating alerts.

Written by the indexing model from the issue text.