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/moderation/moderation.service.ts

@@ -4,6 +4,7 @@ import { DrizzleClient } from "@shared/db/DrizzleClient";
 import type { CreateCaseOptions, ClearCaseOptions, SearchCasesFilter } from "@/modules/moderation/moderation.types";
 import { getUserWarningEmbed } from "@/modules/moderation/moderation.view";
 import { CaseType } from "@shared/lib/constants";
+import { SystemError } from "@shared/lib/errors";
 
 export interface ModerationCaseConfig {
     dmOnWarn?: boolean;
@@ -100,7 +101,7 @@ export const moderationService = {
         });
 
         if (!moderationCase) {
-            throw new Error("Failed to create moderation case");
+            throw new SystemError("Failed to create moderation case");
         }
 
         const warningCount = await getActiveWarningCount(options.userId);

shared/modules/moderation/prune.service.ts

@@ -2,6 +2,7 @@ import { Collection, Message, PermissionFlagsBits } from "discord.js";
 import type { TextBasedChannel } from "discord.js";
 import type { PruneOptions, PruneResult, PruneProgress } from "@/modules/moderation/prune.types";
 import { config } from "@shared/lib/config";
+import { UserError, SystemError } from "@shared/lib/errors";
 
 /**
  * Fetch messages from a channel
@@ -30,7 +31,7 @@ async function processBatch(
     userId?: string
 ): Promise<{ deleted: number; skipped: number }> {
     if (!('bulkDelete' in channel)) {
-        throw new Error("This channel type does not support bulk deletion");
+        throw new UserError("This channel type does not support bulk deletion");
     }
 
     // Filter by user if specified
@@ -54,7 +55,7 @@ async function processBatch(
         };
     } catch (error) {
         console.error("Error during bulk delete:", error);
-        throw new Error("Failed to delete messages");
+        throw new SystemError("Failed to delete messages");
     }
 }
 
@@ -76,12 +77,12 @@ export const pruneService = {
     ): Promise<PruneResult> {
         // Validate channel permissions
         if (!('permissionsFor' in channel)) {
-            throw new Error("Cannot check permissions for this channel type");
+            throw new UserError("Cannot check permissions for this channel type");
         }
 
         const permissions = channel.permissionsFor(channel.client.user!);
         if (!permissions?.has(PermissionFlagsBits.ManageMessages)) {
-            throw new Error("Missing permission to manage messages in this channel");
+            throw new UserError("Missing permission to manage messages in this channel");
         }
 
         const { amount, userId, all } = options;