fix(seed): give admin demo event memberships (scoped menu home-fallback)

Floorplan/dashboard scoped menu resolves the target event from the
user workspaces; admin had zero event_member rows so clicks silently
landed on /home (perceived broken link). Idempotent V51 enrolls admin
as ORGANIZER of e-2026-live and e-2026-smf.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/backend/src/main/resources/db/migration/V51__demo_admin_event_membership.sql

@@ -0,0 +1,16 @@
+-- V51: admin 계정 행사 멤버십 시드 (플로어플랜/대시보드 스코프 메뉴 해소)
+-- 배경: 스코프 메뉴(EVENT_SCOPED_PATH)는 워크스페이스(event_member)로 대상 행사를 해소한다.
+--       admin 계정이 event_member 0건이라 클릭 시 /home 폴백 → "링크 오류"로 인지(소유자 2026-07-14 지적).
+-- 데모 해소행사(e-2026-live)·차기 행사(e-2026-smf)에 admin 을 ORGANIZER 멤버로 등록(멱등).
+INSERT INTO event_member (id, event_id, user_id, role_code, booth_id, company_id)
+SELECT v.id, v.event_id, 'admin-d80a2603', 'ORGANIZER', NULL, NULL
+FROM (VALUES
+        ('em-admin-live', 'e-2026-live'),
+        ('em-admin-smf',  'e-2026-smf')
+     ) AS v(id, event_id)
+WHERE EXISTS (SELECT 1 FROM app_user u WHERE u.id = 'admin-d80a2603')
+  AND EXISTS (SELECT 1 FROM event e WHERE e.id = v.event_id)
+  AND NOT EXISTS (
+        SELECT 1 FROM event_member em
+        WHERE em.event_id = v.event_id AND em.user_id = 'admin-d80a2603'
+      );