feat(items): add event item type (foundations)
Adds 'event' to the items.type allowlist (schema CHECK + ItemType
union + datastore migration for existing databases) and gives it a
calendar emoji icon in card-helpers' TYPE_ICONS. getItemDisplayInfo
recognises type=event and surfaces metadata.startsAt as the subtitle
when present, falling back to "Event". ICS generation and the cal
widget land as separate slices on top of this.