docs: sync from backend 7688f57 — sweep gas strategy: PermitPull + GasTopUp signers

07 - Development/Environment Variables.md

@@ -318,6 +318,14 @@ DERIVED_DESTINATION_MIN_SWEEP_AMOUNT=0
 DERIVED_DESTINATION_SWEEP_INTERVAL_MS=300000
 DERIVED_DESTINATION_SWEEP_AUTOSTART=true
 
+# Master sweep wallet private key (pays gas for permit() + transferFrom() on non-BSC
+# chains; sends BNB gas top-ups on BSC). Should be a dedicated low-balance hot wallet
+# — NOT the same key used for escrow release/refund.
+SWEEP_MASTER_PRIVKEY=
+# BSC gas top-up thresholds (in BNB). If derived address BNB balance is below MIN, top up by TOP_UP.
+SWEEP_GAS_MIN_BNB=0.001
+SWEEP_GAS_TOP_UP_BNB=0.002
+
 # OAuth
 GOOGLE_CLIENT_ID=
 ```