fix(maintenance): auto-expiry + adaptive markdown height + tests

- Auto-expiry: expired events auto-end on GET /events via task manager
- Height: _estimate_markdown_height adapted to unit=8px scale with padding detection
- CRITICAL-1: removed dead import remove_chart_from_position (QA finding)
- CRITICAL-2: added chart alive check in ensure_banner_chart (QA finding)
- CRITICAL-3: fixed test assertions update_markdown_chart → update_banner_on_dashboard
- @POST contract: fixed return range [2,12] → [19,200]
- Tests: 8 new tests (auto-expiry + layout height)

frontend/src/routes/migration/+page.svelte

@@ -492,7 +492,7 @@
 
   <!-- #region TaskHistorySection [C:2] [TYPE Block] [SEMANTICS migration, task-history] -->
   <!-- @BRIEF Section showing recent migration task history. -->
-  <TaskHistory />
+  <TaskHistory pluginId="superset-migration" />
   <!-- #endregion TaskHistorySection -->
 
   <!-- #region ActiveTaskSection [C:2] [TYPE Block] [SEMANTICS migration, active-task] -->