I suggest to have a look to the Discussion The new $system.external interface and the linked .NET samples available in GitHub.

If you want to store FHIR resources you need a FHIR Server, not a gateway, for more info:

Installing a New FHIR Server

Make sure your license include FHIR Server functionality.

Have you tried using the proper, official, documented and supported way to do that using Foreign Tables?

Does the memory leak happen using Foreign Tables ?
Maybe the way you are using the "FOR INTERNAL USE" classes is not proper/correct.

Why using something you are not supposed to use when there is a proper/supported way to do this?

I'm afraid I don't have an answer, I use JDBC regularly (admittedly , not Postgres and NOT using %XDBC classes) and I've never seen this behavior.
If anyone wants to dive deeper in this issue, more information are required to analyze and maybe try to reproduce it, like:

"....rapid increase in memory consumption by the IRIS server" What process is consuming memory? 
What OS, Windows or Linux? If Linux, what flavor?
What Java version are you using?
What version of Postgres  JDBC driver are you using?

Last but to least, note that class %XDBC.Gateway says:
"FOR INTERNAL USE - do not invoke directly"

LoadPageDir() compiles the subdirectories as well if/when in the Web Application definition the setting "Web Settings: Recurse" is flagged (enabled). (It's flagged/enabled by default when creating a Web App)

How is Recurse setting set in your "/CSP/ourapp/" Web Application definition/configuration?

Note that the directory you pass to LoadPageDir is in fact the path of a Web Application, not the physical path on disk.

To reconcile which user/process are associated with the license counts you can use the UserList Query in %SYSTEM.License (aka $system.License) class.

Quick test:

Set rs=$system.License.UserListFunc()
Do rs.%Display()

select * from INFORMATION_SCHEMA.SCHEMATA;

There are many tables in INFORMATION_SCHEMA schema you can query to get info, check:

select * from INFORMATION_SCHEMA.TABLES where TABLE_SCHEMA ='INFORMATION_SCHEMA';

Have you tried explicitly adding to the production a BP using Ens.ScheduleHandler  class?

Make sure the CSPSystem user has read access to the database where your custom login page is located.

https://docs.intersystems.com/iris20253/csp/docbook/DocBook.UI.Page.cls?...

Are you using the IRIS login page or a custom login page defined in the Web Application?

Please define "capture filename", I don't understand.

@Marcelo Witt, what product/version are you using?

The AI BOT answers are moderated, that means that must be approved by a moderator before being published.

I do not understand "That is troublesome, particularly if Vitaliy is a Global Masters user.", can you please elaborate your concern?

OK, thank you, now that you edited adding the link it makes more sense.

Hi @Fan Ji, is the item "The complete FHIR SQL Builder configuration" missing a screenshot?

I would just decode the B64 then if it catch any error or returns nothing ("") is not valid.