refactor: convert LootdropService from class to singleton object pattern

- Move instance properties to module-level state (channelActivity, channelCooldowns)
- Convert constructor cleanup interval to module-level initialization
- Export state variables for testing
- Update tests to use direct state access instead of (service as any)
- Maintains same behavior while following project service pattern

Closes #4

shared/modules/economy/lootdrop.service.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, mock, beforeEach, afterEach, spyOn } from "bun:test";
-import { lootdropService } from "@shared/modules/economy/lootdrop.service";
+import { lootdropService, channelActivity, channelCooldowns } from "@shared/modules/economy/lootdrop.service";
 import { lootdrops } from "@db/schema";
 import { economyService } from "@shared/modules/economy/economy.service";
 
@@ -67,8 +67,8 @@ describe("lootdropService", () => {
         mockFrom.mockClear();
 
         // Reset internal state
-        (lootdropService as any).channelActivity = new Map();
-        (lootdropService as any).channelCooldowns = new Map();
+        channelActivity.clear();
+        channelCooldowns.clear();
 
         // Mock Math.random
         originalRandom = Math.random;