#55 FTX-55: Mail drainer with mint-at-send

Tokens minted at send time in drainer - outbox rows carry NO token material (UB-TOK-4); skip obsolete tasks; one-click minted only when policy allows and no justification (OP-OC-1/AC-8); revoke prior unconsumed one-click; backoff via next_attempt_at + dead state (fixes permanent-fail-on-first-error); mail outage never affects decision state (UN-EML-2, AC-13). Phase 3.