fix: standardize error classes in shared service modules

Replace raw `Error` with `UserError` for user-facing conditions (invalid trade state, user not found, permission/channel type checks) and `SystemError` for internal failures (DB insert failures, external API errors, missing config). Improves Discord UX by ensuring user-facing errors are surfaced cleanly via withCommandErrorHandling.

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

shared/modules/trivia/trivia.service.ts

@@ -3,7 +3,7 @@ import { eq, and, sql } from "drizzle-orm";
 import { config } from "@shared/lib/config";
 import { withTransaction } from "@/lib/db";
 import type { Transaction } from "@shared/lib/types";
-import { UserError } from "@shared/lib/errors";
+import { UserError, SystemError } from "@shared/lib/errors";
 import { TimerType, TransactionType } from "@shared/lib/constants";
 import { DrizzleClient } from "@shared/db/DrizzleClient";
 
@@ -94,13 +94,13 @@ class TriviaService {
             const data = await response.json() as OpenTDBResponse;
 
             if (data.response_code !== 0 || !data.results || data.results.length === 0) {
-                throw new Error('Failed to fetch trivia question');
+                throw new SystemError('Failed to fetch trivia question');
             }
 
             const result = data.results[0];
 
             if (!result) {
-                throw new Error('No trivia question returned');
+                throw new SystemError('No trivia question returned');
             }
 
             // Decode base64