Auto-delete empty ShowEvent after admin ShowEntry delete

[asku1990]

Description

Align admin show delete behavior with admin trial delete behavior.

When deleting a ShowEntry in admin manage flow:

• Keep ShowEntry as the primary delete target.
• After successful delete, count remaining ShowEntry rows for that ShowEvent.
• If remaining count is 0, automatically delete the now-empty parent ShowEvent.
• Do not add a separate “delete ShowEvent” user action.

Scope:

• packages/db/admin/shows/manage/delete-show-entry.ts: add empty-parent cleanup in same transaction.
• packages/server/admin/shows/manage/delete-show-entry.ts: return deletedShowEvent: boolean.
• apps/web/app/actions/admin/shows/manage/delete-admin-show-entry.ts: pass through new response field.
• apps/web/queries/admin/shows/manage/use-delete-admin-show-entry-mutation.ts: keep invalidation; ensure UI handles deleted parent.
• apps/web/components/admin/shows/manage/**: if deletedShowEvent === true, clear selected event / fallback safely.
• Update contracts and local index.ts exports accordingly.

Acceptance criteria

• Admin can delete a single show result row.
• Parent ShowEvent remains when other entries exist.
• Parent ShowEvent is auto-deleted when last entry is removed.
• No boundary violations (business logic in server, writes in db).
• Tests cover success, not_found, event remains, event removed, and UI handling of deleted parent.