refactor: clarify database parameter validation in search service

Add comment to document that filter parameters use pgtype.Text
with explicit Valid=true flag to ensure proper SQL parameter handling.
This clarifies the intent behind the parameter building logic.

Improves code documentation for future maintenance.

internal/services/search.go

@@ -63,7 +63,7 @@ func (s *SearchService) SearchMediaItemsUnified(ctx context.Context, params Sear
 		searchPattern = "%" + searchQuery + "%"
 	}
 
-	// Build database parameters
+	// Build database parameters - only set valid true if filter
 	dbParams := database.SearchMediaItemsUnifiedParams{
 		UserID:         params.UserID,
 		AuthorFilter:   pgtype.Text{String: params.AuthorFilter, Valid: true},