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/vendor/github.com/google/uuid/CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# How to contribute
+
+We definitely welcome patches and contribution to this project!
+
+### Tips
+
+Commits must be formatted according to the [Conventional Commits Specification](https://www.conventionalcommits.org).
+
+Always try to include a test case! If it is not possible or not necessary,
+please explain why in the pull request description.
+
+### Releasing
+
+Commits that would precipitate a SemVer change, as described in the Conventional
+Commits Specification, will trigger [`release-please`](https://github.com/google-github-actions/release-please-action)
+to create a release candidate pull request. Once submitted, `release-please`
+will create a release.
+
+For tips on how to work with `release-please`, see its documentation.
+
+### Legal requirements
+
+In order to protect both you and ourselves, you will need to sign the
+[Contributor License Agreement](https://cla.developers.google.com/clas).
+
+You may have already signed it for other Google projects.