In case someone want to implement the same thing, let me share my solution:
I made * subdomain for each problem, e.g. 1.mydiscourse.com, 2.mydiscourse.com, etc.
Then, Discourse code was modified a bit to parse the subdomain & filter the topics accordingly. The topics Model was also appended with problem_id column.
https://github.com/fikriauliya/discourse/commit/c2654fa8a76edc10209c5ea1bebab9aa344aeadc
Quite minor change, but the downside is that I need to maintain this fork. Would be better if can create plug in (which I am still not sure how).