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`.

packages/shared/src/index.ts

@@ -1,4 +1,5 @@
 export * from "./errors/index.js";
 export * from "./logger/index.js";
-export * from "./types/index.js";
 export * from "./utils/index.js";
+export * from "./moderation-types.js";
+export * from "./config/index.js";