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/render/Dockerfile

@@ -0,0 +1,12 @@
+FROM golang:1.25-alpine AS builder
+WORKDIR /app
+# Dependencies are vendored — build fully offline (proxy.golang.org is geo-blocked from some regions)
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -mod=vendor -ldflags="-s -w" -o render-svc ./cmd/server
+
+FROM alpine:3.20
+RUN apk add --no-cache ca-certificates tzdata
+WORKDIR /app
+COPY --from=builder /app/render-svc .
+EXPOSE 8080
+CMD ["./render-svc"]