refactor(inventory): flatten effects directory structure

Move effect handlers from effects/ subdirectory to flat structure:
- effects/handlers.ts → effect.handlers.ts
- effects/registry.ts → effect.registry.ts
- effects/types.ts → effect.types.ts

Update import path in inventory.service.ts from
'@/modules/inventory/effects/registry' to
'@/modules/inventory/effect.registry'.

This reduces directory nesting and follows the convention of
keeping module files flat unless there are 5+ files.

bot/modules/inventory/effect.registry.ts

@@ -0,0 +1,20 @@
+import {
+    handleAddXp,
+    handleAddBalance,
+    handleReplyMessage,
+    handleXpBoost,
+    handleTempRole,
+    handleColorRole,
+    handleLootbox
+} from "./effect.handlers";
+import type { EffectHandler } from "./effect.types";
+
+export const effectHandlers: Record<string, EffectHandler> = {
+    'ADD_XP': handleAddXp,
+    'ADD_BALANCE': handleAddBalance,
+    'REPLY_MESSAGE': handleReplyMessage,
+    'XP_BOOST': handleXpBoost,
+    'TEMP_ROLE': handleTempRole,
+    'COLOR_ROLE': handleColorRole,
+    'LOOTBOX': handleLootbox
+};