S17 Phase 1: repository read endpoints (list, detail)

GET /api/v1/:slug/repositories (filter by format and kind, keyset pagination, sort) and GET /api/v1/:slug/repositories/:repository_name (polymorphic settings). Returns counters as stored (the buffered-counter mechanism is S18/S22). Only hosted rows exist in Phase 1; the kind filter is defined now and returns remote and virtual repositories once S13 lands. Unblocks the frontend repository read epic (&22321). Follows the merged Phase 1 plan.