refactor: modernize transaction patterns and improve type safety

- Refactored user.service.ts to use withTransaction() helper
- Added 14 comprehensive unit tests for user.service.ts
- Removed duplicate user creation in interactionCreate.ts
- Improved type safety in interaction.routes.ts

src/events/interactionCreate.ts

@@ -48,13 +48,9 @@ const event: Event<Events.InteractionCreate> = {
 
         // Ensure user exists in database
         try {
-            const user = await userService.getUserById(interaction.user.id);
-            if (!user) {
-                console.log(`🆕 Creating new user entry for ${interaction.user.tag}`);
-                await userService.createUser(interaction.user.id, interaction.user.username);
-            }
+            await userService.getOrCreateUser(interaction.user.id, interaction.user.username);
         } catch (error) {
-            console.error("Failed to check/create user:", error);
+            console.error("Failed to ensure user exists:", error);
         }
 
         try {