refactor: standardize error handling with toError() helper

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/orchestrator.ts

@@ -1,4 +1,5 @@
 import { nanoid } from 'nanoid'
+import { toError } from '../lib/utils.js'
 import { db, schema, sqlite } from '../db/index.js'
 import { eq, sql } from 'drizzle-orm'
 import { randomArena, type Arena } from './arenas.js'
@@ -207,7 +208,7 @@ async function callWebhook(
   } catch (err: unknown) {
     const elapsed = Date.now() - start
     const isAbort = err instanceof Error && err.name === 'AbortError'
-    console.log(`[webhook] ${url} ${isAbort ? "TIMEOUT" : "ERROR"} in ${elapsed}ms: ${err instanceof Error ? err.message : err}`)
+    console.log(`[webhook] ${url} ${isAbort ? "TIMEOUT" : "ERROR"} in ${elapsed}ms: ${toError(err).message}`)
     return {
       answer: null,
       timeMs: elapsed,