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>

.env.example

@@ -1,4 +1,13 @@
-# FlatRender Admin API (optional — set to enable dynamic templates/categories)
+# ── FlatRender V2 API Gateway ────────────────────────────────────────────────
+# Single public entrypoint to all microservices. Auth + data flow through this.
+# Local dev: gateway on :8088 (GATEWAY_PORT in .env.v2). In Docker: http://gateway:8080
+API_GATEWAY_URL=http://localhost:8088
+# Client-side base (future direct data calls). Include the /v1 prefix.
+NEXT_PUBLIC_API_URL=http://localhost:8088/v1
+# Tenant the public site authenticates against (Identity service).
+NEXT_PUBLIC_TENANT_SLUG=flatrender
+
+# FlatRender Admin API (LEGACY V1 — being replaced by the gateway above)
 # Run the admin-api service at D:\Projects\flatrender-admin\admin-api
 # Leave empty to use hardcoded fallback data
 ADMIN_API_URL=http://localhost:5000