Scheduler Event Timeline Lazy Loading

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

• Server-backed schedules too large to hold entirely in memory
• Refetching when the user pages the timeline or toggles filters

• Enable SchedulerFeatures::LAZY_LOADING and SchedulerFeatures::TIMELINE in features.
• Pass data_source: SchedulerDataSourceMode::Remote(...) implementing SchedulerDataSource.
• Bind events as RwSignal<Vec<PlannedEvent>> — the data source writes into this signal.
• Bump lazy_reload_key to force a refetch after external filter changes.