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/vitest.config.js

@@ -13,7 +13,8 @@ export default defineConfig({
       'src/**/*.{test,spec}.{js,ts}',
       'src/lib/**/*.test.{js,ts}',
       'src/lib/**/__tests__/*.{test,spec}.{js,ts}',
-      'src/lib/**/__tests__/test_*.{js,ts}'
+      'src/lib/**/__tests__/test_*.{js,ts}',
+      'tests/**/*.test.{js,ts}'
     ],
     exclude: [
       'node_modules/**',