feat: add CRUD operations for notifications_sent table

Implement MarkNotificationSent, IsNotificationSent, and
GetUnnotifiedReleases with LEFT JOIN query. Includes 10
table-driven tests covering success, error, idempotent,
and edge cases. Also go fmt cleanup on existing files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/database/artist_settings_test.go

@@ -225,7 +225,7 @@ func TestUpdateArtistSettings_Success(t *testing.T) {
 
 	// Update only name and monitored.
 	updates := map[string]interface{}{
-		"name":     "Updated",
+		"name":      "Updated",
 		"monitored": false,
 	}
 	if err := UpdateArtistSettings(db, "artist-1", updates); err != nil {