#69 FTX-69: Concurrency + isolation E2E packs

Concurrency: double-approve, one-click double-consume, approve-vs-expire, n_of_m triple race (dedup_key uniqueness proves single terminal side effect), 50-way idempotent create. Isolation: tenant-B key against every tenant-A surface (REST/MCP/tokens) -> 404/403 no existence leakage (AC-1). Phase 9.