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/database/index.ts

@@ -16,7 +16,7 @@ export async function initializeDatabase() {
 
   const databaseUrl =
     config.DATABASE_URL ||
-    `postgresql://${config.DATABASE_USER}${config.DATABASE_PASSWORD ? `:${config.DATABASE_PASSWORD}` : ""}@${config.DATABASE_HOST}:${config.DATABASE_PORT}/${config.DATABASE_NAME}`;
+    `postgresql://${config.POSTGRES_USER}${config.POSTGRES_PASSWORD ? `:${config.POSTGRES_PASSWORD}` : ""}@${config.POSTGRES_HOST}:${config.POSTGRES_PORT}/${config.POSTGRES_DB}`;
 
   pool = new Pool({
     connectionString: databaseUrl,