feat: implement background payment sync worker and admin trigger UI

components/checkout/CryptoCheckout.tsx

@@ -108,6 +108,29 @@ export default function CryptoCheckout({ amount, currency, txId, wallets, onSucc
         setSelectedToken(network.tokens[0]);
     };
 
+    // Save payment intent to DB so background sync knows what to check
+    useEffect(() => {
+        if (!txId || !selectedNetwork || !selectedToken) return;
+        
+        const saveIntent = async () => {
+            try {
+                await fetch(`/api/transactions/${txId}/intent`, {
+                    method: 'POST',
+                    headers: { 'Content-Type': 'application/json' },
+                    body: JSON.stringify({
+                        network: selectedNetwork.id,
+                        token: selectedToken.symbol
+                    })
+                });
+            } catch (err) {
+                console.error("Failed to save payment intent:", err);
+            }
+        };
+
+        const timer = setTimeout(saveIntent, 1000); // Debounce
+        return () => clearTimeout(timer);
+    }, [txId, selectedNetwork.id, selectedToken.symbol]);
+
     const handleCopy = () => {
         navigator.clipboard.writeText(depositAddress);
         setCopied(true);