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/modules/ai-moderation/concurrencyLimiter.ts

@@ -0,0 +1,14 @@
+import pLimit from "p-limit";
+import { config } from "../../shared/config/config.js";
+
+/**
+ * Concurrency limiter for LLM API calls.
+ *
+ * Prevents rate-limit (429) errors by capping simultaneous requests
+ * to the configured maximum (default: 5).
+ */
+const llmSemaphore = pLimit(config.AI_LLM_MAX_CONCURRENT ?? 5);
+
+export async function withLlmConcurrency<T>(fn: () => Promise<T>): Promise<T> {
+  return llmSemaphore(fn);
+}