Implement expiry-based query methods on CertificationRepository

Use Supabase PostgREST filters directly: .gte('expires_at', DateTime.now().toIso8601String()) and .lte('expires_at', DateTime.now().add(Duration(days: withinDays)).toIso8601String()). For getCertificationsExpiringThisMonth, compute the last day of the current month in Dart using DateTime(now.year, now.month + 1, 0) and pass it to getExpiringSoon. Avoid calling getExpiringSoon(90) followed by client-side filtering for getExpiringSoon(30) — each method should issue its own targeted query. Add a brief JSDoc-style comment above each method explaining its use case (cron job vs.

dashboard widget) to guide future maintainers. Consider adding a stream-based variant (watchExpiringSoon) later using Supabase Realtime for live dashboard updates.

Unit tests (flutter_test with mocked Supabase client): verify that getExpiringSoon(30) constructs a query with gte(now) and lte(now+30d) filters; verify that getExpiredCertifications constructs a lt(now) filter on expires_at. Integration tests against local Supabase: seed 5 active, 3 expiring-in-7d, 2 expired, and 1 suspended certification; assert each method returns the expected subset. Test that getExpiringSoon(0) returns an empty list (no certifications expiring in zero days). Test that the method handles a null expires_at correctly (certification with no expiry should appear in getActiveCertifications but not in getExpiringSoon).