refactor(web): move timeline-manager ops back into month-group; clean up API

Consolidates asset operation logic within TimelineManager class and removes the now redundant 
operations-support.svelte.ts file. 

Combines addAsset/updateAsset to be upsertAsset. 


Changes:
- Move `addAssetsToMonthGroups` logic into TimelineManager's `addAssetsToSegments`, `upsertAssetIntoSegment`, `postCreateSegments`, and `postUpsert` methods
- Move `runAssetOperation` from operations-support into TimelineManager's private `#runAssetOperation` method
- Rename public `addAssets`/`updateAssets` methods to unified `upsertAssets` for consistency
- Delete internal/operations-support.svelte.ts
- Update WebsocketSupport to use `upsertAssets` for both add and update operations
- Fix AssetOperation return type to allow undefined/void operations (not just `{ remove: boolean }`)
- Update MonthGroup constructor to accept `loaded` parameter for better initialization control
- Update all test references from `addAssets`/`updateAssets` to `upsertAssets`

This refactoring improves code maintainability by eliminating duplicate logic and consolidating all asset operations within the TimelineManager class where they belong.

web/src/lib/utils/actions.ts

@@ -109,5 +109,5 @@ export function updateUnstackedAssetInTimeline(timelineManager: TimelineManager,
     },
   );
 
-  timelineManager.addAssets(assets);
+  timelineManager.upsertAssets(assets);
 }