fix(migration): deduplicate active invites before creating unique index in V026

Dev databases that accumulated multiple pending invites before V026 was
written would fail to create uq_household_invite_active. Added a cleanup
UPDATE that marks all-but-the-latest invite per household as invalidated
before the index is created.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V026__add_invite_invalidated_at.sql

@@ -1,6 +1,17 @@
 ALTER TABLE household_invite
     ADD COLUMN invalidated_at timestamptz;
 
+-- Mark all but the most-recent invite per household as invalidated,
+-- so the unique partial index below can be created on dev databases
+-- that accumulated multiple pending invites before this migration was added.
+UPDATE household_invite
+SET    invalidated_at = NOW()
+WHERE  id NOT IN (
+    SELECT DISTINCT ON (household_id) id
+    FROM   household_invite
+    ORDER  BY household_id, expires_at DESC
+);
+
 CREATE UNIQUE INDEX uq_household_invite_active
     ON household_invite (household_id)
     WHERE invalidated_at IS NULL;