Annotations and links on schedule items

[japsu]

Currently, annotations/cachedAnnotations and links are only available on ProgramItem.

However, we have several valid use cases that necessitate having them at least under the hood in ScheduleItem:

• signing up to schedule items independently (especially Konsti)
• enabling & configuring Paikkala for schedule items independently

The SQL field annotations already exists on ScheduleItem but is not used in any way.

Annotations have the is_applicable_to_program_items/schedule_items flags. Annotations with both should probably be pushed down from ProgramItem to ScheduleItem.

[japsu]

Annotations on schedule items now exist in API/DB but no UI. Used via setup script for flea market reservations via Paikkala in Tracon 2025.

cachedAnnotations(ownOnly: true) returns cached_annotations, without returns cached_combined_annotations. Push-down implemented via Program.refresh_dependants.

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