fix: commit default campaign claim in ensure_admin_exists, surface setup errors to user

backend/app/auth.py

@@ -102,7 +102,8 @@ def ensure_admin_exists():
     # Claim the "default" campaign for the admin so setup wizard works
     admin = db.get_user_by_username(ADMIN_USERNAME)
     if admin:
-        conn.execute(
-            "UPDATE campaigns SET created_by = ? WHERE id = 'default' AND created_by IS NULL",
-            (admin["id"],),
-        )
+        with db.tx():
+            conn.execute(
+                "UPDATE campaigns SET created_by = ? WHERE id = 'default' AND created_by IS NULL",
+                (admin["id"],),
+            )