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)

backend/src/core/database.py

@@ -21,6 +21,7 @@ from ..models import (
     config as _config_models,  # noqa: F401
     dataset_review as _dataset_review_models,  # noqa: F401
     llm as _llm_models,  # noqa: F401
+    maintenance as _maintenance_models,  # noqa: F401
     profile as _profile_models,  # noqa: F401
     task as _task_models,  # noqa: F401
 )