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>

infra/docker/Dockerfile.backend

@@ -0,0 +1,29 @@
+FROM node:20-alpine
+
+WORKDIR /app
+
+# Install pnpm
+RUN npm install -g pnpm
+
+# Copy workspace files
+COPY pnpm-workspace.yaml .
+COPY pnpm-lock.yaml .
+COPY package.json .
+
+# Copy shared package
+COPY packages/shared ./packages/shared
+
+# Copy backend service
+COPY services/backend ./services/backend
+
+# Install dependencies
+RUN pnpm install --frozen-lockfile
+
+# Build backend
+RUN pnpm run build:server
+
+# Expose port
+EXPOSE 3001
+
+# Start backend
+CMD ["node", "dist/index.js"]