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/tests/test_layout_utils.py

@@ -0,0 +1,55 @@
+# #region test_layout_utils [C:3] [TYPE TestModule] [SEMANTICS test, layout, superset, height, estimation]
+# @BRIEF Contract tests for layout utility functions — _estimate_markdown_height.
+#   Verifies the height estimation formula: empty → 19, short text → computed,
+#   padding accounted for, and long text capped at 200.
+# @LAYER Test
+# @RELATION BINDS_TO -> [LayoutUtils]
+# @TEST_CONTRACT: _estimate_markdown_height(str) -> int in [19, 200]
+# @TEST_EDGE: empty_content -> returns minimum height (19)
+# @TEST_EDGE: html_only_content -> returns minimum height (19)
+# @TEST_EDGE: content_with_padding -> padding:12 correctly increases height
+# @TEST_EDGE: very_long_content -> capped at maximum height (200)
+import pytest
+
+from src.core.superset_client._layout_utils import _estimate_markdown_height
+
+
+class TestEstimateMarkdownHeight:
+    """Tests for _estimate_markdown_height pure function."""
+
+    # #region test_empty_content [C:2] [TYPE Function]
+    # @BRIEF Empty string returns minimum height 19.
+    def test_empty_content(self):
+        assert _estimate_markdown_height("") == 19
+    # #endregion test_empty_content
+
+    # #region test_single_line_text [C:2] [TYPE Function]
+    # @BRIEF Short text without padding returns expected computed height.
+    #   200 chars → 200//40+1 = 6 lines → 6*22 = 132px → 40+132 = 172px → 172//8 = 21
+    def test_single_line_text(self):
+        text = "A" * 200
+        assert _estimate_markdown_height(text) == 21
+    # #endregion test_single_line_text
+
+    # #region test_content_with_padding [C:2] [TYPE Function]
+    # @BRIEF Content with padding:12 — 24px added to content height.
+    #   200 chars + padding:12 → 6*22 + 24 = 156 → 40+156 = 196 → 196//8 = 24
+    def test_content_with_padding(self):
+        content = '<div style="padding:12">' + "A" * 200 + "</div>"
+        assert _estimate_markdown_height(content) == 24
+    # #endregion test_content_with_padding
+
+    # #region test_very_long_content [C:2] [TYPE Function]
+    # @BRIEF Very long content (3000 chars) capped at max height 200.
+    def test_very_long_content(self):
+        text = "A" * 3000
+        assert _estimate_markdown_height(text) == 200
+    # #endregion test_very_long_content
+
+    # #region test_html_only_content [C:2] [TYPE Function]
+    # @BRIEF HTML-only content (no visible text) returns minimum height 19.
+    def test_html_only_content(self):
+        content = "<br/><hr/><div></div>"
+        assert _estimate_markdown_height(content) == 19
+    # #endregion test_html_only_content
+# #endregion test_layout_utils