Implement threshold deduplication logic in NotificationRecordRepository

For `hasNotificationBeenSentForThreshold`, use: `.from('notification_records').select('id').eq('recipient_id', recipientId).eq('threshold_days', thresholdDays).eq('notification_type', notificationType).limit(1)` — checking list.isNotEmpty is more reliable than COUNT in the Supabase Dart client. For `recordThresholdNotificationSent`, use Supabase's `.upsert(payload, onConflict: 'recipient_id,notification_type,threshold_days,reference_id', ignoreDuplicates: true)` — the `ignoreDuplicates: true` flag means a conflict is silently ignored, achieving idempotent behavior safe for concurrent edge function invocations. For `getUnreadCount`, the Supabase Dart client supports `count: CountOption.exact` on a `.select()` call: `.from('notification_records').select('*', const FetchOptions(count: CountOption.exact)).eq('recipient_id', recipientId).eq('is_read', false)` — access the count via `response.count`. This avoids fetching full row data for a badge count.

Document the service-role vs. authenticated-client usage differences in code comments at the top of the concrete class.

Unit tests with mocked Supabase client: (1) hasNotificationBeenSentForThreshold returns true when mock returns count > 0; (2) returns false when mock returns count = 0; (3) recordThresholdNotificationSent calls upsert with ignoreDuplicates=true or equivalent and does not throw on conflict; (4) getUnreadCount returns 0 when mock returns empty result; (5) getUnreadCount returns correct integer from count response. Integration test against local Supabase: (1) call recordThresholdNotificationSent twice with same args — confirm only one record exists in DB; (2) confirm hasNotificationBeenSentForThreshold returns true after first call; (3) mark the record as read and confirm getUnreadCount decrements. Test concurrent calls by calling recordThresholdNotificationSent from two Future.wait parallel calls and assert single DB record.