The wishlist list endpoint previously loaded and JSON-decoded the full
wishlist, filtered by category in Python, then sliced in memory. Cost
grew linearly with wishlist size on every page request.
get_wishlist_tracks now accepts offset and category parameters, both
applied in SQL via LIMIT/OFFSET and json_extract. get_wishlist_count
also accepts category so COUNT(*) matches the filtered page. The API
endpoint uses these to return only the requested page.
Backward compatible: other callers (core/wishlist_service) pass no
offset/category and still receive the full list.
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