Centralizes all 31 direct users-table SQL queries from 11 handler
files into a single UserRepository, following the same pattern as
CharacterRepository and GuildRepository. The only excluded query is
the sign_sessions JOIN in handleMsgSysLogin which spans multiple
tables.
Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.
