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.

shared/modules/inventory/inventory.service.ts

@@ -171,7 +171,7 @@ export const inventoryService = {
             const results: any[] = [];
 
             // 2. Apply Effects
-            const { effectHandlers } = await import("@/modules/inventory/effects/registry");
+            const { effectHandlers } = await import("@/modules/inventory/effect.registry");
 
             for (const effect of usageData.effects) {
                 const handler = effectHandlers[effect.type];