refactor(setup): derive setup-complete status from admin user count

Setup completion was previously tracked by a manually-flipped setup_complete row in system_settings, written via a JWT-protected PUT /api/setup/complete endpoint. This meant any admin user created outside the setup wizard (future CLI, seed scripts, direct DB inserts) would not flip the switch, leaving the app stuck redirecting to /setup.

The trigger is now derived from real data: setup is complete iff at least one admin user exists. This is self-correcting regardless of how users are created, and re-engages setup automatically if all admins are ever removed.

Changes:
- Add internal/setupstatus package with IsSetupComplete() (queries CountAdmins, 10s in-memory cache, fails open on DB error) and Invalidate() to clear the cache. Uses an AdminCounter interface to avoid importing the database package.
- Add CountAdmins sqlc query (SELECT COUNT(*) FROM users WHERE role = 'admin') and regenerate.
- Rewire router/setup.go isSetupComplete() to delegate to setupstatus; drop the old setup_complete setting read, cache vars, and the PUT /api/setup/complete route.
- Call setupstatus.Invalidate() in the auth handler after CreateUser, UpdateUserRole, and DeleteUser so the cache reflects admin-count changes immediately.
- Align first-user promotion in Register to key off !adminExists instead of len(users) == 0, so the two checks cannot diverge.
- Remove the now-dead SetSetupComplete/GetSetupStatus handlers.
- Drop the setup_complete seed row from schema.sql.
- Remove the apiPut('/setup/complete') call from the setup wizard finishSetup(); the admin account created in submitAdmin already marks setup complete server-side.

internal/database/queries.sql.go

@@ -226,6 +226,17 @@ func (q *Queries) ClearKoboShelfByName(ctx context.Context, arg ClearKoboShelfBy
 	return err
 }
 
+const CountAdmins = `-- name: CountAdmins :one
+SELECT COUNT(*) FROM users WHERE role = 'admin'
+`
+
+func (q *Queries) CountAdmins(ctx context.Context) (int64, error) {
+	row := q.db.QueryRow(ctx, CountAdmins)
+	var count int64
+	err := row.Scan(&count)
+	return count, err
+}
+
 const CountUnlinkedBooks = `-- name: CountUnlinkedBooks :one
 SELECT COUNT(*) as count
 FROM unlinked_books