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/message-capture/pagination.ts

@@ -0,0 +1,21 @@
+export interface CursorData {
+  created_at: number;
+  id: string;
+}
+
+export function encodeCursor(data: CursorData): string {
+  return Buffer.from(JSON.stringify(data)).toString("base64");
+}
+
+export function decodeCursor(cursor?: string): CursorData | null {
+  if (!cursor) return null;
+  try {
+    const data = JSON.parse(Buffer.from(cursor, "base64").toString("utf-8"));
+    if (typeof data.created_at === "number" && typeof data.id === "string") {
+      return data;
+    }
+    return null;
+  } catch {
+    return null;
+  }
+}