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/api/routes/settings.py

@@ -393,6 +393,7 @@ class ConsolidatedSettingsResponse(BaseModel):
     storage: dict
     notifications: dict = {}
     features: dict = {}
+    app_timezone: str = "Europe/Moscow"
 
 
 # #endregion ConsolidatedSettingsResponse
@@ -463,6 +464,7 @@ async def get_consolidated_settings(
             storage=config.settings.storage.dict(),
             notifications=notifications_payload,
             features=config.settings.features.model_dump(),
+            app_timezone=config.settings.app_timezone,
         )
         logger.reflect(
             "Consolidated settings payload assembled",
@@ -526,6 +528,15 @@ async def update_consolidated_settings(
             payload["notifications"] = settings_patch["notifications"]
             config_manager.save_config(payload)
 
+        # Update app_timezone if provided
+        if "app_timezone" in settings_patch:
+            new_tz = settings_patch["app_timezone"]
+            from ...core.timezone import validate_timezone, invalidate_timezone_cache
+            if not validate_timezone(new_tz):
+                raise HTTPException(status_code=422, detail=f"Invalid IANA timezone: {new_tz}")
+            current_settings.app_timezone = new_tz
+            invalidate_timezone_cache()
+
         config_manager.update_global_settings(current_settings)
         return {"status": "success", "message": "Settings updated"}