fix: null-Guard für log.data + Fallback-Warnung im RPC-Client (Review Task 4)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/server/signals/onchain.ts

@@ -30,7 +30,7 @@ export function decodeTransferLogs(logs: any[]): OnchainTransfer[] {
   for (const log of logs) {
     const token = TOKEN_BY_CONTRACT.get(String(log.address).toLowerCase());
     if (!token) continue;
-    const raw = BigInt(log.data === '0x' ? '0x0' : log.data);
+    const raw = BigInt(!log.data || log.data === '0x' ? '0x0' : log.data);
     out.push({
       symbol: token.symbol,
       instrument: token.instrument,
@@ -59,6 +59,7 @@ export async function rpc(method: string, params: unknown[]): Promise<any> {
       return json.result;
     } catch (err) {
       lastErr = err;
+      console.warn(`RPC ${method} auf ${url} fehlgeschlagen — nächster Endpunkt:`, err instanceof Error ? err.message : err);
     }
   }
   throw lastErr;