Support categories for in-house apps self-service

[jkatz01]

Fleet version:

Web browser and operating system:

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💥  Actual behavior

Software categories are not supported for in-house apps, they can be clicked on in the UI but do not save.
https://fleetdm.slack.com/archives/C086V2QK76X/p1762807444348879?thread_ts=1762807426.362499&cid=C086V2QK76X

🛠️ To fix

Support categories for in-house apps in the backend

🧑‍💻  Steps to reproduce

1. Edit in-house app, add some categories, then observe the change is not saved when you come back.