[US-043] Super admin: create and manage categories

[leimo2011]

Epic: EP-08 — Super Admin Dashboard \nPriority: Must Have (M) \nStory Points: 3 \nSprint: Sprint 1\n\n### User Story\nAs a super admin, I want to manage (create/edit/delete) categories so the taxonomy stays relevant.\n\n### Acceptance Criteria\n- [ ] Super admin can create new categories with name, slug, icon\n- [ ] Categories can be edited and soft-deleted (deactivated)\n- [ ] Parent category support for sub-categories\n- [ ] Deactivating a category hides stores from that category in browse\n- [ ] Categories table shows store count per category\n\n### Technical Notes\n- GET/POST /api/admin/categories/\n- PATCH/DELETE /api/admin/categories/{id}/\n- is_active flag for soft delete