refactor(shared): centralize configuration and types

Migrate configuration validation and core moderation types from individual services to the `@bete/shared` package to ensure consistency across the monorepo.

- Move `AppConfig` and moderation-related interfaces to `packages/shared`.
- Replace service-specific Zod schemas with the centralized shared configuration.
- Refactor `services/backend` and `services/discord-gateway` to consume shared config and types.
- Remove redundant type definitions and local configuration logic in services.
- Update `packages/shared` exports to include new `config` and `moderation-types` modules.
- Clean up unused files and deprecated utility functions in `packages/shared`.

services/backend/src/shared/middlewares/index.ts

@@ -60,7 +60,7 @@ export function requireParam(
   name: string,
 ): string {
   if (typeof value !== "string" || value.length === 0) {
-    throw new Error(`Missing ${kind}: ${name}`);
+    throw new ValidationError(`Missing ${kind}: ${name}`);
   }
   return value;
 }