test(database): #351 group-assignment regression test is integration-tagged, may not run in default CI

[cristim]

Discovered while auditing PR #533 (which closes #351).

Two follow-ups from the #533 review. The PR's runtime behaviour correctly satisfies #351 (group_ids seeded on both INSERT paths, idempotent backfill via assignAdminGroupAndWarn, WARN on residual drift), so this is a hardening follow-up, not a regression of the fix itself.

1. Regression test does not run in standard CI. The only test, TestEnsureAdminUser_GroupAssignment in internal/database/postgres/migrations/ensure_admin_user_test.go, carries //go:build integration and needs a running postgres container. A standard go test ./... run (without -tags integration) skips it entirely, so the fix(auth): bootstrap admin must be auto-assigned to the Administrators group (ensureAdminUser gap) #351 security-relevant invariant has no protection in the default CI pipeline. Confirm the CI workflow runs the integration suite; if it does not, either add an integration-test job or add a non-integration unit test that exercises the backfill SQL against an in-memory or mocked layer.

2. Backfill lives in Go, not in a migration. Issue fix(auth): bootstrap admin must be auto-assigned to the Administrators group (ensureAdminUser gap) #351 acceptance criterion 2 asks for the backfill in migration 000024 (or an extension), idempotent. PR fix(auth): auto-assign bootstrap admin to Administrators group (closes #351) #533 instead backfills in assignAdminGroupAndWarn, invoked only when RunMigrations is called with a non-empty admin email. A DB restored from a backup, or migrated without ADMIN_EMAIL set, would not get the backfill applied to pre-existing drifted admin rows. A SQL-level idempotent backfill (in a new migration) would close that path. Decide whether the Go-level backfill is sufficient for the deployment model or whether a migration-level backfill is warranted.

Acceptance

• The fix(auth): bootstrap admin must be auto-assigned to the Administrators group (ensureAdminUser gap) #351 group-assignment invariant is covered by a test that runs in the default CI pipeline.
• Decision recorded on whether a migration-level idempotent backfill is needed for restore/no-admin-email paths.

[cristim]

Verification: shipped in PR #579 (SHA acd9873, merged 2026-05-28). migration 000056_backfill_admin_group_ids.up.sql added with idempotent SQL UPDATE backfill for admin users with empty group_ids. Decision recorded: SQL-level backfill closes the backup-restore / no-ADMIN_EMAIL path. Confirmed on origin/feat/multicloud-web-frontend.