refactor: split monolith into 3 microservices (frontend, backend, discord-gateway)

- Extract services into services/{frontend,backend,discord-gateway}
- Create packages/shared/ for shared logger, errors, utils, types
- Setup Modular MVC pattern in backend (controller→service→repository)
- Setup event-driven architecture in discord-gateway with Redis pub/sub
- Move Docker files to infra/docker/ with per-service Dockerfiles
- Update docker-compose.yml to use Traefik-only routing (no port exposes)
- Update GitHub Actions deploy workflow for multi-service matrix build
- Fix all import paths and resolve type errors across all services
- All 3 services pass tsc --noEmit clean

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,65 @@
+export class AppError extends Error {
+  constructor(
+    message: string,
+    public code: string,
+    public statusCode: number = 500,
+  ) {
+    super(message);
+    this.name = "AppError";
+  }
+}
+
+export class ValidationError extends AppError {
+  constructor(
+    message: string,
+    public details?: Record<string, unknown>,
+  ) {
+    super(message, "VALIDATION_ERROR", 400);
+    this.name = "ValidationError";
+  }
+}
+
+export class NotFoundError extends AppError {
+  constructor(message: string) {
+    super(message, "NOT_FOUND", 404);
+    this.name = "NotFoundError";
+  }
+}
+
+export class UnauthorizedError extends AppError {
+  constructor(message: string = "Unauthorized") {
+    super(message, "UNAUTHORIZED", 401);
+    this.name = "UnauthorizedError";
+  }
+}
+
+export class ForbiddenError extends AppError {
+  constructor(message: string = "Forbidden") {
+    super(message, "FORBIDDEN", 403);
+    this.name = "ForbiddenError";
+  }
+}
+
+export class ConflictError extends AppError {
+  constructor(message: string) {
+    super(message, "CONFLICT", 409);
+    this.name = "ConflictError";
+  }
+}
+
+export class DatabaseError extends AppError {
+  constructor(
+    message: string,
+    public originalError?: Error,
+  ) {
+    super(message, "DATABASE_ERROR", 500);
+    this.name = "DatabaseError";
+  }
+}
+
+export class ConfigError extends AppError {
+  constructor(message: string) {
+    super(message, "CONFIG_ERROR", 500);
+    this.name = "ConfigError";
+  }
+}