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/discord-gateway/src/shared/errors/errors.ts

@@ -0,0 +1,43 @@
+export class AppError extends Error {
+  public code: string;
+  public statusCode: number;
+
+  constructor(message: string, code: string, statusCode: number = 500) {
+    super(message);
+    this.code = code;
+    this.statusCode = statusCode;
+    this.name = "AppError";
+    Error.captureStackTrace(this, this.constructor);
+  }
+}
+
+export class ConfigError extends AppError {
+  constructor(message: string) {
+    super(message, "CONFIG_ERROR", 500);
+    this.name = "ConfigError";
+  }
+}
+
+export class AudioError extends AppError {
+  constructor(message: string) {
+    super(message, "AUDIO_ERROR", 500);
+    this.name = "AudioError";
+  }
+}
+
+export class VoiceConnectionError extends AppError {
+  constructor(message: string) {
+    super(message, "VOICE_CONNECTION_ERROR", 500);
+    this.name = "VoiceConnectionError";
+  }
+}
+
+export class ValidationError extends AppError {
+  public details?: Record<string, string[]>;
+
+  constructor(message: string, details?: Record<string, string[]>) {
+    super(message, "VALIDATION_ERROR", 400);
+    this.details = details;
+    this.name = "ValidationError";
+  }
+}