feat: V2 microservices stack — backend services, gateway, JWT auth

Add full V2 architecture: identity, content, studio (.NET 10) and file,
render, notification, gateway (Go) services with vendored deps, plus DB
migrations, event/API contracts, and an init-db script.

Wire the Next.js frontend to the gateway: server-side JWT auth routes
(login/register/refresh/logout/me), gateway fetch helper, and session/
cookie/jwt helpers under src/lib.

Containerize the stack via docker-compose.v2.yml and per-service
Dockerfiles. Base images resolve through a Nexus mirror (Docker Hub) and
MCR directly; npm/NuGet pull from Nexus groups. Self-host fonts via
next/font/local to avoid Google Fonts (geo-blocked).

Add CI workflow and ignore .env.v2, *.stackdump, and .NET bin/obj.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

services/gateway/docker-compose.yml

@@ -0,0 +1,16 @@
+services:
+  gateway:
+    build: .
+    ports:
+      - "8080:8080"
+    environment:
+      JWT_SECRET: "${JWT_SECRET}"
+      IDENTITY_URL: "http://identity-svc:8080"
+      CONTENT_URL: "http://content-svc:8080"
+      FILE_URL: "http://file-svc:8080"
+      STUDIO_URL: "http://studio-svc:8080"
+      RENDER_URL: "http://render-svc:8080"
+      RENDER_WS_URL: "ws://render-svc:8080"
+      NOTIFICATION_URL: "http://notification-svc:8080"
+      PORT: "8080"
+    restart: unless-stopped