#79 FTX-79: Retention worker (P2)

Daily via Redis SET NX marker; terminal decisions past RETENTION_DAYS=365 (tenant override clamped >=30) batch-deleted child-first 1000/txn; audit pass; platform audit summary per run (UB-NF-6, UB-DM-5); prohibition tests: no tenant DELETE routes (405), UN-AUD-1. Phase 7.