Scheduler Event Calendar Lazy Loading

Load calendar events from a remote source for the currently visible range.

• Server-backed schedules that are too large to preload entirely
• Refetching when the user changes week or toggles filters

• Enable SchedulerFeatures::LAZY_LOADING in features.
• Pass data_source: SchedulerDataSourceMode::Remote(...) implementing SchedulerDataSource.
• Bind events: RwSignal<Vec<PlannedEvent>> — fetched rows merge into this signal.
• Increment lazy_reload_key to trigger a refetch after external changes.