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/scripts/seed_superset_load_test.py

@@ -148,20 +148,10 @@ def _resolve_target_envs(env_ids: list[str]) -> dict[str, Environment]:
     resolved: dict[str, Environment] = {}
 
     if not configured:
-        for config_path in [Path("config.json"), Path("backend/config.json")]:
-            if not config_path.exists():
-                continue
-            try:
-                payload = json.loads(config_path.read_text(encoding="utf-8"))
-                env_rows = payload.get("environments", [])
-                for row in env_rows:
-                    env = Environment(**row)
-                    configured[env.id] = env
-            except Exception as exc:
-                logger.reflect(
-                    f"Failed loading environments from {config_path}: {exc}",
-                    extra={"src": "_resolve_target_envs"},
-                )
+        logger.reflect(
+            "No environments found in DB — seed script requires environments configured via API first",
+            extra={"src": "_resolve_target_envs"},
+        )
 
     for env_id in env_ids:
         env = configured.get(env_id)