refactor: break monorepo into 3 standalone services (gateway, backend, frontend)

- Remove pnpm workspace, moon repo, and all monorepo tooling
- Delete packages/shared/, embed shared code directly into each service
- Copy packages/shared/src/* -> services/backend/src/shared/ and services/discord-gateway/src/shared/
- Replace all @bete/shared imports with @/shared/ path alias
- Remove @bete/shared workspace dependency from both services
- Update root package.json scripts from --filter to --prefix
- Rewrite Dockerfiles to build each service standalone
- Clean up biome.json, .gitignore, remove root drizzle.config.ts

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

@@ -0,0 +1,48 @@
+// Custom error classes for all services
+
+export class AppError extends Error {
+  constructor(
+    message: string,
+    public code: string,
+    public statusCode: number = 500,
+    public details?: Record<string, unknown>,
+  ) {
+    super(message);
+    this.name = "AppError";
+  }
+}
+
+export class ValidationError extends AppError {
+  constructor(message: string, details?: Record<string, unknown>) {
+    super(message, "VALIDATION_ERROR", 400, details);
+    this.name = "ValidationError";
+  }
+}
+
+export class NotFoundError extends AppError {
+  constructor(resource: string, id?: string) {
+    super(`${resource} not found${id ? `: ${id}` : ""}`, "NOT_FOUND", 404);
+    this.name = "NotFoundError";
+  }
+}
+
+export class UnauthorizedError extends AppError {
+  constructor(message = "Unauthorized") {
+    super(message, "UNAUTHORIZED", 401);
+    this.name = "UnauthorizedError";
+  }
+}
+
+export class DatabaseError extends AppError {
+  constructor(message: string, details?: Record<string, unknown>) {
+    super(message, "DATABASE_ERROR", 500, details);
+    this.name = "DatabaseError";
+  }
+}
+
+export class ConfigError extends AppError {
+  constructor(message: string) {
+    super(message, "CONFIG_ERROR", 500);
+    this.name = "ConfigError";
+  }
+}