List unsubscribe groups

Offset pagination only (page[number] default 1, page[size] default 25, max 100). No sort/filter/cursor — ordering fixed desc created_at, desc id. Uses the shared QueryParser.parse_offset/1 — a malformed page (e.g. flat ?page=1 instead of ?page[number]=1) returns 400 (code 10015), consistent with GET /v2/email_blocks. meta includes total_pages.