Implements GuildService.BreakupAsync (soft-delete guild, hard-delete dependents, clear viewer GuildId pointers). Wires /guild/breakup, /guild/emblem_list, /guild/others_info in GuildController. Fixes ToDetailDto to accept and populate leader_name from the member list. Adds GuildServiceBreakupTests (3 tests: cascade delete, permission guard, leader_name HTTP regression). Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>