feat(setup): seed default categories with subcategories and demo content (#141)
Community initialization now creates a default category tree with 4 root
categories (General, Development, Community, Feedback) and 7 subcategories
(Frontend, Backend, DevOps, Showcase, Events, Bug Reports, Feature Requests).
Also seeds 7 demo topics (one per leaf category) with one reply each,
using the admin's DID as author. Topics include realistic content so the
forum feels alive from the first visit.
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